Motherboards come in a few form factors, pick the motherboard you want, and get a case that is the same form factor that fits your needs, the looks you want, and the budget. ATX is full sized, mini-ATX is smaller, ITX is more of the trinket size fit in a model computer (not something you really need to worry about), BTX doesn't seem to be too common yet. Some cases come with power supplies, some of them are junk, some are fantastic (the Antec cases typically come with a good Antec power supply). If it comes with a power supply, just read the reviews for it to see what people think of the power supply, or look up the power supply separately and see what people think of it in those reviews. Look around on Newegg too for their bundle deals, you can often get a price break buying certain things together (case & motherboard, motherboard & CPU).
