How do you lock on?

Lock on? if you mean like aim at a person and keep the aim on them when they move.
then really i think it means like to hold it steady or your gun wont move when you shoot

You can't log on in PC!! But you can log on in Playstation2. In PC You Just Aim.

I mean Lock on.

Lock-on aiming in possible in GTA SA on pc. Pause the game, go to options, choose controller options, you'll see the configuration. Change it from mouse+keys to joypad (remember that ps2 uses a joypad in laying its controls). You can still use everything but the mouse for camera controls. Turning the camera is placed on the NUM controls 2,4,6,8. That should so the trick.

maybe you should try a game pad, sevenpanda.com have a lot of game pads, I recommend that to you.---_sevenpanda.com/products_new.html?page=5&cate=88&disp_order=6