You will find a little icon below everybody's avatar:
Click on this when you like or dislike someone's post. It will either add or subtract to that person's reputation. You are able to leave a comment about the post, to let the person know why you are giving them or docking them rep.
You will see little green boxes underneath the avatar:
These are reputation boxes... The more positive rep you have, the more of these green boxes will be displayed... The more negative rep you have, the more red boxes will be displayed.
To see the rep and the comments that people are leaving for you, simply click on the "UserCP" button in the menu bar at the top of the screen... Currently, it is anonymous. I'm not certain that I will keep it anonymous... But I just don't know how to change it right now, so it'll have to remain anonymous until I figure it out.