When you choose wheels, two things matter: size and hardness.
Size
Wheel size is given in mm. Bigger wheels make you go faster. They generally weigh a bit more and sit your board higher off the ground. As a rule, bigger wheels are chosen by transition skaters (from 55mm up) and smaller wheels by street skaters (50 to 54mm). If you’re a beginner, go for a wheel between 52 and 55mm; those are common sizes. Keep in mind that you’ll want higher trucks with bigger wheels (from 55mm up).
Hardness
Wheel hardness is given in A values. A high A value means a hard wheel, a low A value means a soft wheel. Most skaters prefer slightly harder wheels because you feel the surface you’re rolling over. Harder wheels also let you powerslide and slide better on the obstacles you trick on. That’s not just fun, it can make landing a trick a little more forgiving too. Softer wheels are mainly good for rougher ground, where they soak up the bumps in the surface. As a beginner, go for something around 101A. For a cruiser, aim for something around 78 to 90A.
A few brands use B values instead. To get the A value of a wheel marked in B, add 20 to the A value. For example: 76A is 96B, and 100B is 80A.