Hackole

So basically I have to be lower than my target to intercept?

So I've had KSP for awhile now and decided to make a refueling station so I can get to the mun. Problem is, when I get the two ships into the same orbit, I can't figure out how to get them next to each-other. If I burn prograde and speed up to meet the other ship, the orbit gets bigger, and a similar thing happens when I try to slow down. So I have no clue how to get them even near each-other .