MAXOFLIFE:
How do I know what kind of transistor works in a certain situation?
Another part needed: A bit of math. Here are a couple of links to get you started (note, this is for bipolar transistors - mosfets are a whole 'nother game):
By no means is the above a complete set of equations, etc needed, but it is enough to get started with. When you move into larger power transistors (TO-220 and TO-3 case devices), you'll also need to get into power dissipation equations (and how to properly size a heat sink for proper current handling). The equations aren't very hard, but they do require a bit of thought and understanding about how things work and how the devices are constructed (to understand certain terms).