Depends on which skin you are using. Scroll down to the bottom of your screen and on the very lower left corner of the page, it will give you 2 choices: AZ VB3 style (the old style which I prefer) or Tech Two.
If it is Tech Two, look for some little buttons underneath the message window of a post that says rep with a blue line and bad with a red line. In the other style, click on the scales in the upper right of the post window blue border. It is between the post # and the red triangle. Clicking on those will take you to another window that asks if you approve (green) or disapprove )red) of the post. You can write in a little message if you want, then send it.
If you are pretty new, your reps will be gray, but the gesture is still appreciated, especially if there is a nice note to go along with it. Pretty soon, you will build up a nice row of greenies.
Go to User CP to find our who has left reps for you.