you need to click the silver button above any post to be able to rep anyone....
As people keep repping you, you get more and more green bars.
But the person who repped you should have a few rep points to start off with.
For example, you have 0 rep points. If you rep someone, it will not make any difference to their reputation.
If people with more reputation rep you, the more points you get. For example, if I rep you, you will get some amt of reps. But say someone else with lesser rep than me reps you, then you will get lesser reps from them than you got from me