Brahmanda Purana and Varaha Purana establishes timeline of Sri Venkateswara Swamy at Tirumala hills since the start of creation on earth.
After Varaha (in the form of Elasmotherium) rescued land from water, he asked Garuda to bring a hill from Vaikuntha and drop it on earth at a place chosen by him.
Garuda brings in Kreedachala (an extensive natural hill with lofty peaks, embedded with gold and precious stones, and which resembled Sesha in shape) from Vaikuntha.

This spot was the first place that emerged out of water after Varaha killed Hiranyaksha.
Varaha appeared with four arms, a white face and resolved to stay at this hill.
Present kalpa is called Sweta Varaha Kalpa on his name.
Brahmanda Purana mentions that once Narada was asked by Sriman Narayana for a good place to stay on earth.
Narada suggests Venkatachala as people in that place were leading their lives with no interest on Vedas and need the blessings of the Lord. He prayed to Lord that he should reside in that place along with Goddess Lakshmi to save these people of Kaliyuga.
Narayana asked Adisesha to proceed to the earth and remain in the form of a mountain range at the place mentioned by Narada.
The mountain range from Tirumala till SriSailam called black hills in Nallamalla range, resemble a snake with hood at Tirumala.

Natural Arch also called Silathoranam, which means rock garland is naturally formed in the quartzites.
Vishnu put his first foot down at a place called Sreevari paadalu (Divine foot prints) which is the highest point of Tirumala hills, the second step at the location of the arch.

The height of rock arch is same as main idol inside the temple. It is believed that he entered through this arch.

Thereafter, the next step is stated to have been placed where his idol is now worshipped in the temple at Tirumala.

After many years, Lord Garuda had a thought to take Venkatachala back to Vaikunta. On perceiving this, Lord Narayana said, “Oh king of birds! We shall all continue staying on this mountain. You stay in the southern part of the mountain. Sesha, you become mountain beneath Garuda. I will stay here to protect the world until the dissolution.
In each Yuga, this mountain will be known with a different name. In Kruta Yuga, it is Vrushadri, Thretha Yuga Garudachala, Dwapara Yuga Seshadri, Kali Yuga
Venkatachala. Various sages and devotees will also call this mountain as Narayanadri across different yugas.”

Kumara Swamy (Subrahmanya or Kartikeya), after killing Tarakasura, was sent to these hills by Siva to get rid of ‘Brahma Hatya Patakam‘.
He did penance and bathed in pushkarini, a pond from where Vishnu appeared later.
The first one to get rid of his sins by worshipping Vishnu on this hill is Kumara Swamy and God here was called Venkateswara, which means removal of sins.

Long after Kartavirya Arjuna, his descendant in Haihaya dynasty, King Sankhana ruled from his capital Satrupura. His father’s name was Sruta.
He lost kingdom due to treachery of his vassal kings and was wandering with wife, son and ministers.
He visited Rameswaram in south and then proceeded towards Sri Kalahasti. He gets a message to do penance on bank of Swami Pushkarini on Narayanadri (tirumala) with the wish of seeing his divine form for six months.
After six months, SankhaNa sees a divya vimana emerging from middle of Pushkarini with Vishnu in standing position with all weapons, Sri Devi and Bhu Devi.

Brahma and other devas also visited that place along with Vishnu, who grants Sankhana his kingdom.
Sankhana returns to his kingdom to find that, vassal kings killed each other out of greed to become emperor.
People accept Sankhana back as their ruler of the kingdom on banks of river Godavari.

Fist temple in Tirumala of Venkateswara Swamy

After few years, Sankhana was ordered by Vishnu to make an idol in exact form which emerged from Pushkarini and build a temple around it.

यथादृष्टं विमानमते मदरूपं पुरुषर्षभ ।
तथाह मिहा भागम ते नते भविता गति:।।

Sankhana crowned his son as king and returned to tirumala, where he made the first known Venkateswara Swamy idol along with Gopura and other smaller idols.
This is how worship started in tirumala.