When it comes to choosing wheels, There are 2 things you should be aware of. Hardness and size.

Size:

50-53mm:  Small, slow wheels; Perfect for tricks and smaller skaters, skaterparks and bowls
54-59mm: Average wheel size for beginners and larger riders; Perfect for street, park, bowls and vert.
60-69mm: Cruiser / penny board, Freeride / Freestyle longboards and old-school riders; made to be fast and harder surfaces.
70-80mm:  Large wheel for rough surfaces; Perfect for downhill longboarding and old-school riders.

Hardness:

78a-87a: Soft wheels good for rough surfaces, longboards or streetboards, that need plenty of grip for easy rolling over cracks and pebbles. Designed to run long trips, cruising, longboards, downhill and rough surfaces.
88a-95a: Slightly harder and faster with slightly less grip. Good for cruising and street driving on rough surfaces.
96a-99a: Good speed and grip - an all-around good wheel. Great for beginners who skate street, park, ramps, pools and other slippery surfaces.
100a +: The hardest and fastest wheel with the least possible grip. Ineffective on smooth and rough surfaces. These are pro-wheels.