Skateboard wheels come in a variety of colours, sizes and degrees of hardness. There are two main things to think about when buying your first set of wheels...
From Sidewalk Basics - Autumn 2013
Diameter, this relates to how tall the wheel is. This is measured in millimetres (mm) for skateboard wheels.
Durometer, this relates to how hard the wheel is. Most skateboard wheels use what's called the "a-scale". You don't need to understand all the details of how the a-scale is measured, just that the higher the number, the harder the wheel.
Most skaters will be happy with wheels from 52mm to 54mm, with a hardness of around the 99a mark.
You’ll need a set of 8 bearings for your wheels. Each wheel houses two bearings on either side of the wheel’s core. Most skaters insert a spacer in between each bearing to maximise their performance. There are loads of different bearings on the market, the more expensive ones offer much higher performance but some of the generic cheaper sets are fine for a beginner. Ask your local skate store for advice on this.