Installing rear wheel:

Put the right side shifter in high gear position. Put the top section of chain at the derailleur onto the smallest rear sprocket. Slip the wheel into position and tighten the axle nut and quick-release lever. The left slot (called "dropout") is often longer than the right because the derailleur mounting bracket occupies part of the right dropout (slot). The axle will not go all the way in the left dropout. This is normal.

As you gradually tighten the wheel into position, make sure it is centered between the brake, and also the front of the chain stays. These are the frame tubes that run from the dropouts to the bottom bracket housing which holds the crank.