Well it depends on the position... You have to be able to see the weaknesses in the opponent's position and find a way to gain some kind of advantage off of it. Sometimes it's tactical and you have to find some sharp moves or sack material and other times you end up developing differently than usual to target the weaknesses. Sometimes there's nothing special to do but you get a nice position out of the opening. But it's not really any different than any other position. There may just be other rare features like kings in the center. Usually when something is possible you have to change plans and don't necessarily follow rules or principles. It depends...
Here are some games that show typical tactics in the opening:
http://www.chessgames.com/perl/chesscollection?cid=1006294
They start out simple and get more complicated. You don't have to look at them all.
I play in sub-1400 games online and don't study openings. I suspect most of my opponents at these levels don't study openings much either (although I could be wrong!).
I mean, I'll study the first 6-7 moves of my favorite openings, but not 20 moves deep and certainly not multiple variations at those depths.
I simply rely on opening principles + some basic shallow opening knowledge to try and get to a safe middle-game.
However, I've wondered how to punish opening weaknesses after playing for a while? Most chess tutorials show how to open properly and assume the opponent will too. But none that I've seen show how to punish improper openings.
I'm wondering if people can post games where opponents have made bad opening moves and how to best punish those moves? We could create a running thread if people are interested. Thanks very much!