Now that you have the basics of exposure down, you should start honing those skills of composition. With enough practice, the two will become like second nature without you having to think too much about it. But for now, the only way to get to that point is to practice.
Nikon has a great website with an introduction and plenty of examples:
Add interest by framing

Here are a few more examples of framing from my past photo shoots:

Jen Rhoades Photography, copyright 2011
Jen Rhoades Photography, copyright 2011