Can anyone help me to set up my keyboard to play Freespace 2 .I would like to use the WSAD for moving and the mouse for fireing.

Given that FS2 is a flight simulator, using the mouse only for firing, doesn't make much sense... also, if i'm not mistaken, you can pitch your fighter in the 4 main direction and bank left or right (that is, rotate on your axis, which you're not gonna use that often in combat), but there is no strafe, so you can't map the A and D button in the usual way you expect

True that. You can check out the current keymapping by pressing F2 (or going to the Options menu from the mainhall), and then clicking on "Control Config". But, as the previous poster said, FS2 is a flight sim, it needs a slightly different control setup compared to a FPS.
Also, it's a really, really bad idea to skip the tutorial missions.