The 6 pointed star on the lower left hand side of each post is the rep button. (Next to "Blog this Post") You don't need any rep points to rep someone else. But if you rep someone, then you will find you cannot rep them for a while, until you rep other people.
I hope that explains the basic principle of repping!
PS. The triangle with the exclamation point next to the rep button if for reporting people who have violated forum rules.