how do you set the aperture?
Read the manual.... then read some basic photography books (try National Geographic Field Guide to Photography)... then read the manual again. This will help you much more than any forum answer can.
For easy and high quality pictures indoors - with the gear you already have - try this:
Set the camera to "M" (manual) mode by turning the top left dial to "M". Turn both dials (top/right front and rear/back) until the numbers say 125 (for 1/125 of a second) and 5.6 (representing an aperture of f/5.6). Press the little button on top of the camera that says "Drive/ISO" and turn the dial until the ISO reads 200 or 400. Press the little button on the top of the camera that has a little lightning bolt and a +/- sign and then adjust the rear dial until the marker reads somewhere between +1/3 and +1. Now take photos - point the flash straight up at a white ceiling for a more natural lighting effect. If there are no white ceilings, point the flash straight at the person.
Hope this helps!