I've noticed many posts on various forums similar to the following...
"OMGWTF I have a bow.. now what do I do with it!?" 
Well, I'm here to help!
Choose a suitable build.I'm in favor of pure str or str-based hybrids for bow, but that's just me.
I don't recommend pure int for bow because pure ints have nukes, which are already long range attack. They also have low life and a bow does not offer any defense, so you're more susceptible to getting owned.
Feel free to play around with builds a bit to see which you're most happy with and settle with that.
Which element?Cold is popular among bowmen. It'll slow the enemy down so that you'll may be able to kill them before they even touch you. STRs can also go fire. However, it is entirely up to you and what you feel most comfortable with.
don't be a thief at lvl 20, you'll die a lot from the high levels. Ice has been nerfed so go for fire/light sub.