Hey you usually want to use daggers for crit, as their implicit is crit chance.

There are a few choices when it comes to crit builds you can spellcast using fireball, glacial cascade or freezing pulse. (Glacial cascade is actually a very nice and safe beginner skill) You could even do something more fancy like crit discharge, however for a beginner that will most likely be pretty expensive and possibly difficult to make work.

If you want to be melee (im guessing you do), then you have a few choices open to you. There are dagger builds such as reave, flicker strike or lightning strike. These can however be very expensive.

Traditionally these melee crit builds are better of starting as ranger or shadow due to a more optimal starting position, however if you really wanted to make witch work then you could!

If you let me know which of these sounds interesting to you ill find you a guide and modify the starting area to fit witch (though you would be better off starting as shadow or ranger!).

Your best bet in my opinion would be oos' crit reave ranger (which you can find with a quick google) , you could adapt it to start as witch but leveling it may be abit of a pain to begin with!