To set aperture on a D5100 switch to A and use the command dial.
(Shooting in Manual mode is simply torturing yourself unnecessarily if you don't know exactly what you're doing. In Aperture priority — or Program shift — setting aperture is simple and intuitive.)
My D7000 is more straightforward since it has a separate dial for this and no need to half depress the shutter release. (I assume you can also change the settings using the LCD, but that's of no use if you're looking through the viewfinder and trying to set manual exposure.)
Edit: A mode on Canons is Av mode.
I started off with P&S cameras and before I knew the mechanics of cameras I was frustrated with the automatic exposures; once I discovered I was missing "manual mode" I never turned back.
Sure, from time to time I'll use the priority or program modes when the shot is more important than the learning experience, but after 4 years of hobbyist photography I still find myself with much to learn and I enjoy the challenge of manual exposures.