It's well covered in this thread: http://en.thewitcher.com/forum/index.php?/topic/30828-how-does-rep-work/
Make posts that people enjoy or find helpful, and they'll vote your reputation up. Spam, troll, or flame, and they'll vote you down.
Mind the tooltips on the "+" and "-" signs at the bottom right of each post; they're wrong, and if you follow the tooltips and not the signs, you'll vote down instead of up, or up instead of down.