I have a nikon d3100 and i can't find how to change the fstop?

If anybody knows how and could tell, me id appreciate it!

Look on pages 73 to 77

All you need to know are in those pages

Download your manual from Nikon.
http://www.nikonusa.com/...100_EN.pdf

Then start reading on page 76.

Hint: f/stop = aperture

Set to A mode then turn the wheel. In M mode, press +/- before turning the wheel. You can't change it in S mode.