How do you attack in assassins creed 2 on PC

It depends on your intended method. Generally speaking though you need to look towards the target so there is a white outline around them and press F on your keyboard. Then press and hold Q on your keyboard and choose your intended weapon (e.g. long sword) by moving the mouse over it. Then walk or run towards the subject and press W to hit them with the weapon. Use right mouse button for offensive actions combined with keyboard commands.

Alternatively you can attack them from above with the hidden blade by selecting it (Q), selecting the taget and pressing W and you will leap down on them from above. I find this particularly effective especially for killing Brutes. Instant kill.