You can find the answer in the second book of Virgil's Aeneid. But tldr:

The Trojans thought that the greeks have left, they went to their camps and found a huge wooden horse. Some wanted to bring the horse into the city, others wanted to burn it, others still wanted to open the stomach and see what was inside, since they thought that it was a trap. A man called Lacoön rushed and yelled at them that the greeks would never gift them anything (that's where the famous saying "Beware of Greeks bearing gifts" comes from), and he threw a spear at the horse, and they heard that something was inside.

But then, some Trojans brought forth a Greek captive named Sinon. He told them that the Greeks wanted to leave for ages, but the winds wouldn't let them. To appease the gods for good winds, they have chosen to sacrifice him, but he somehow got away. The horse was built to atone for the sins commited by Diomedes and Odysseus (they stole a statue of Athena), and it was built higher than the Trojan walls to prevent the Trojans from bringing the horse into the city, for if they bring it in, Troy will conquer Greece.

Then two giant snakes appeared, killed Lacoön and his kids, and then they slithered away to hide behind a statue of Athena. The Trojans understood this as a punishment for Lacoön since he threw a spear at the horse, and that they needed to bring the horse back to the temple of Athena. They then demolished parts of their gate to make room for the horse, heard metal ringing from the inside as they were pulling it, but didn't think much of it. They celebrated the end of the war, went to sleep at nightfall and the rest is ~~history~~ mythology.