This thread:
Reputation
helps quite alot, but to save you sifting through loads of pages.
If you look at the posts, to the left hand side you have that target, a set of scales and a warning sign, if you click the scales, a box pops up, you choose whether its positive or negative, but use the negative one wisely, not just because you disagree with the comment
type on your message and a box will come up confirming it
The more posts you have the more rep you give in one hit, the more green blocks you have the more your rep is worth, but it is truely worth looking through that thread if you have time
edit: pffffft beaten to it