There are a few things you should consider when purchasing a microwave: the size of the unit, the features it has, and how much power it has.
If you're looking for a small microwave to use in a dorm room or office, a countertop model is a good option. If you need a larger microwave, consider a built-in model that will fit into your cabinets.
Some microwaves come with special features, like convection cooking or sensor cooking.