There's a little button at the bottom of each post that looks like this:
When you see a post that you find particularly good or helpful, you can click on that button and bump up the member's rep. Leaving a comment about why you liked their post is nice, too.
You can also choose to take away a rep point if you feel a member has been particularly nasty or mean, but this option should be utilized very carefully. If abused, rep-giving privileges can be taken away.