Can someone suggest me what kind of wepon i should have? - im lvl 15 and for now i have warhammer - min attk: 132.... i alsow have crossbow: min attk: 40

or should i just go with the one that is stronger?

Whoa! Those two are along completely different lines. There are alot of opinions on this but here are some quick overviews.

-mageish should almost always use staves, the attack of damage doesn't matter, go with most int and end, depending on your preference.
-fighterish should either go with something like your warhammer or two faster 1 handed weapons. I personally prefer 2 handed weapons, but the overall DPS doesn't seem huge if the weapons are equal.
-rangerish should pretty much use the best crossbow or gun the find, most people seem to prefer faster weapons but the mythic plate piercer is a notable exception.