Your reputation power is posted under your name and avatar for all to see. It is a measure of how much you can affect someone elses reputation, and is affected by length of membership, number of posts etc...
You give other people good or bad reputation points by clicking on the silver scales at the bottom of the post you wish to comment on.
To find your own reputation points (which is different from your power), either click on the silver scales in one of your own posts, or click on the "User CP" in the panel menu at the top of the page and you will see how many reputation points you have.