This achievement can be achieved very early on in the game. Just follow these steps to get it:

1) Speak to Cortana.
2) Immediately after you finish speaking to Cortana outside the cryotube, go down the hallway.
3) To your left there will be some stairs heading down.
4) Go down the stairs.
5) You will find a computer.
6) Interact with the computer.

It will proceed to show you a few cut scenes explaining the Halo story and an achievement will pop up.