Assassin's Creed II/Walkthrough
The game begins right where Assassin's Creed left off, which is in Abstergo as Desmond Miles. A little backstory is given through a cut scene, and when you gain control, you'll need to break out of Abstergo with Lucy.
Press a button when given the option to begin controlling the game, then follow Lucy to the Animus and press a button to get into it. Lucy does a data search between yourself and subject 16 to find the next DNA sequence for you to explore. A match is found and you witness the birth of a little Italian baby. Follow the on-screen directions to control the different parts of the baby's anatomy. These controls are the same ones that control Ezio throughout the game.
When you exit the Animus, follow Lucy again. When you get down to the large Animus bay on a lower floor, follow Lucy through the machines, stopping when she does to avoid guards. If you run into any, just repeatedly tap the armed hand button to fight. If Lucy's with you, she'll also fight alongside you. When you get to the locked keypad, press any button to use your eagle vision to figure it out.
Down in the parking lot you'll encounter a group of bad guys you need to take out. You can't lose any health at this point and your only fighting moves are punching and blocking. Keep attacking with Lucy until they're all down, then continue following her to her car. She'll take you to the Assassins' base of operations and explain why they're fighting before asking for you to help them. Here you'll meet two more people who will help you throughout the game, Shaun Hastings and Rebecca Crane. Shaun will do historical research for you while you're in the Animus and Rebecca runs the machine. You can talk to the three of them to get some more story information, and you have to talk to each of them at least once before Rebecca will let you into the Animus to start completing the DNA sequences.