Sacred texts and other main texts: The Vedic Revelations
The textual foundations of the Hindus religiosity are found in the corpus called Veda (literally [sacred] science). Originally they were a large set of doctrines, composed roughly around the 1500–1000 BCE. orally transmitted and put in written form only in recent times (I sec. BCE). They are considered not human creation, and supposed to have been directly revealed to the Rishi (sages, or seers) by the gods themselves. They are conceived as revealed truths (Shruti). For this reason, also their language is considered sacred.
Their most ancient core comprise four collections:1) the Rgveda, or Hyms, which celebrate the various deities;2) the Samaveda, or Melodies, which are musical elaborations of the Hyms to be chanted during the rituals; 3) the Yajurveda, or sacrificial formulas and 4) the Atharvaveda, or incantations, the collection less relevant for the sacrificial rite.
The other texts of the Vedic Revelations comprise the Brahmana and Arankya, which are liturgical commentaries of the four collections cited above.
Much later (500 BCE) are the Upanishad texts, which are conceived as the conclusion of the Veda, but also as their final purpose, because they offer the philosophical speculation around the previous vedic texts. This speculation forged the classic indian worldview. For this reason scholars tend to see in the Upanishads a watershed that bridge the Vedic religion to Hinduism.

Deities, holy beings: The Vedic Pantheon
The four collections describe an ancient religious practice focused on the ritual sacrifice to evoke the gods and ask them benefits such as wealth, descendants, power, even immortality. The pantheon comprise a number of deities whose characteristic are not always strictly differentiated.
They are divided in two types: Deva and Ashura, the latter being the most ancient deities who bear occult powers. Later on in the development of Hinduism they will become more similar to demons.
Around 33 deities are numbered in the RgVeda. Some of them are personification of element, like Vayu (the wind) or Visavat (the shining one, epithet of the sun). Indra, the warrior gods, is the "national hero" of the vedic civilization. His victory over a dragon recall the subjugation of the former inhabitants of the Indian subcontinent by the Indo-Aryan, the tribes native of central Asia which invaded the Indu valley and gave birth to the Indian civilization.
Among other prominent devas there is Purusha, the Primordial Man, who is sacrificed and dismembered by the gods themselves in order to create the world and the order of society.
Finally, one of the most important deities is Agni. He is the god of fire (cfr. latin ignis) and the acceptor of sacrifices. The sacrifices made to Agni goes to the deities because Agni is a messenger from and to the other gods.
Even if this pantheon could be assimilated to the Greek counterpart, there's a important difference, see below "Intercultural & Interdisciplinary information".

Main rites: the vedic sacrifice
As it can be easily understood, the sacrifice hold a pivotal role in the origin of the Hindus religions.
The vedic rites can be distinguished in two types: solemn and domestic. The former involves the participation of a number of priests, proportionally to the importance of the occasion and the benefits requested. The sacrifice of the horse, celebrated by a monarch in order to sanction his status of universal ruler, lasted a year. The domestic rites, on the contrary, could be officiated directly by the head of the household.
The vedic sacrifice involved the immolation of a sacrificial victim on the fire altar (symbol of Agni). Its meaning is not limited to the mere request of mundane benefits, but is a more profound one: the ritual patron, that is, the person who pays the officiants and provide the sacrificial victim, ideally sacrifice himself through the medium of the sacrificial victim. Thus he reenact the myth of the Purusha (see above), whose sacrifice created the world. In other words the sacrifice is also conceived as recreating every time the cosmic order, or Rta (cfr. latin ritus).
The Ancient Vedic ritual, though being the embryo of the present Hindus religiosity, is nowadays almost extinct as religious practice. Its importance and influence, however, is still represented in the various contemporary Hindus rituals, like marriage, where a fire altar is always present.

Authority: The Brahmins
Brahmins refers to an individual belonging to the Hindus priest, artists, teachers, technicians class and also to an individual belonging to the Brahmin caste into which an individual is born. Its etymology is linked to the word Brahman, which refers to the creative aspect of the Absolute or Supreme God. They are the priest responsible for religious rituals in temples and homes and they are the person authorized after rigorous training in Vedas and religious rituals to provide advice and impart knowledge of the Absolute to members of the society. The English word Brahmin is an anglicized form of the Sanskrit word Brahmana, liturgical commentaries cited above.
Their symbolic color is white, expressing purity.

Contextualization and analysis of the sources.
Source n° 1
The Rgveda Hyms :Rgveda I.1
It is the first hymn of the Rgveda collection, and the fact that Agni is the first deity cited denotes his importance. This hymn speaks clearly of how Agni can bring fortune to man by functioning as a messenger between humans and gods. Agni is not only the personification of fire, but he also symbolizes the function of the priest.
The 8th verse indicate also the fundamental meaning of the sacrifice, that is: to maintain the cosmic order. As the ruler of sacrifice Agni is therefore also the guard of cosmic order.

Source n° 2
The Rgveda Hyms : Rgveda I, 164
See the section "Intercultural & Interdisciplinary information."

The Fire Ritual
Additional information: the persons with white robes are the Brahmins; the Soma is a mythical drink said to induce visions of the gods.
Commenter: " More than 3000 years ago, nomads entered in India. They spoke an indoeuropean language called vedic. They composed the four Vedas which, later, Hinduism regarded as a sacred revelation. The oldest of these is the Rgveda. Unlike the Bible, which means literally "the Books", the Vedas were handed down without writing, word for word, from father to son, from teacher to pupil. Vedic Nomads used perishable materials such as clay, woods and grass. Agni, god of fire, was kept permanently alive. On the move, he was carried in pots. At rest, in steeles or altars. Fire was ritually celebrated in ceremonies such as the Agnistoma, or "praise of Agni". With the development of Hinduism, the vedic rituals decline. Considered extinct since the Middle Ages, these rituals of sacrifice has survived in remote corners. The ritual patron or sacrificer, with the help of Agni and Vedic mantras, asks the god for strength, wealth, children and immortality. His measurement give a unit of length which determines the size of the altar and its sacred enclosure. This is the origin of geometry in India. The vedic enclosure is always a temporary structure. unlike the later temple of Hinduism. Large altars were made of bricks. The dimensions of squares, rectangles and triangles are also functions of the size of the sacrificer. The altar of fire's bricks can be traced back to the Indus valley civilization, which is earlier than the Vedas. Around the 10th century BCE, the Yajurveda speaks of altar of fire built from  1000 bricks. The altar's construction in a shape of bird is called Agnicayana, the "piling up of Agni". The rituals connected with this construction continue through the night, from which the name of Athirathram or "overnight", derives. One end of the sacred enclosure represent the home of the sacrificer. Simple altars are made of clay. A round altar represents a kitchen fire. A semi-circular altar faces south. Next to the sacrificial altar, ritual implement are kept."
Bramin: "According to Rgveda, the man who performs Agnistoma and Athirathra goes up to heaven, as a result of drinking soma. [...] "we drank soma, then [...] we became [...] (that is) without death". "This is the belief that we, Nambutiri, transmit."
Commenter: " A brahmin is eligible for the Soma ritual only if he keeps three sacred fires in his home throughout his life. The three fires are carried to the sacred enclosure together with wooden blocks and a drill for making new fire. The three fires are installed temporarily. To the west, the domestic fire, the Gharapatya, or domestic altar, is used to cook the oblation. To the south the southern fire, the Daksinagni, or southern altar, to ward off evil influences. To the east, the sacrificial fire, the Ahavaniya, or sacrificial altar, which is used to make oblation. All rites are performed in behalf of the sacrificer, or Yajmana. At the offer called [vedic name], priests call the gods down to earth to partake the oblation. The offerings are poured into the fire, and Agni transmits them to the gods.

Intercultural & Interdisciplinary information.
(History)
The Vedic Pantheon could suggest that the ancient Indian spirituality was polytheistic (that is: comprising a number of discrete gods), like the Greek pantheon. However, as the hym in source 2 affirms, there is a higher reality behind the multiplicity of gods, and that the sages know that all the gods are just different names for a same entity.
For this reason, scholars prefer the term Henotheism. This term was first coined by Max Müller's (1823-1900) one of the first orientalist. Henotheism is the belief in and worship of a single god while accepting the existence or possible existence of other deities that may also be worshipped. In the case of the Vedic religion, it seems that the function of Supreme God is interchangeable between the various divinities, which are considered as various facets, or hypostasis, of a Supreme Being. This will be an important feature in the development of the concept of Ishvara, Supreme God in the subsequent Hinduism.