If they're the Original Poster in a thread, you click the "Thanks" button. Otherwise use the reputation button (that star at the bottom of their avatar).
Or just go to their profile and post on their wall.
You can + rep them.
It's the button that looks like a sheriff budge under your ava and status.Click "I approve" to give them positive rep.
Or you can thanks on a thread they made.That's the smiling face that says thanks under your ava and status again.
To have a rep power you have to reach 50 posts first!