>>96057711
>shooting a stationary target
Hit the DV given in the range table for the weapon you're using (p. 173). If they can dodge bullets, beat their Evasion check (p.172)
>shooting a guy you know is on the other side of drywall
Drywall is considered thick plaster cover, so it has 15 hp (p.182-p.183). Beat the DV table for the range of the cover; you must reduce the cover to 0 hp before you can target them as above.
>but what if the cover is thin?
Thin plaster is not considered cover and has 0 hp, so you can shoot right through it above. That said, if it still blocks line of sight, I would impose a conditional modifier of -1 (equivalent shooting in darkness) on at least the first shot (p.130), as while you know the target is behind the wall, you're still blind firing.
>shooting a guy who is hiding behind a desk
See above. A desk is considered thin wood cover so it only has 5 hp (p. 183), but it'll probably block at least one shot.