To pre-empt the less than helpful types who are going to rush in here and say "google sheldon brown", there's a couple basic things that might help you on your way.

Different length BB's give you different chainlines. You need to find the right combination of BB length, crank, and rear wheel/cog to get a straight chainline. Read about chainlines.

Crank length is a personal preference. Longer cranks give you more torque, but are a little bit more difficult to spin with (meaning pedal as quickly as you can at very high RPM) and can be susceptible to pedal strike. The longer the pedal, the closer to the ground it will be. If you lean far enough into a turn, the pedal can hit the ground, and you will be sad.
Shorter cranks are the opposite; easier to spin, less fear pedal strike, but less leverage.
I use 165mm, which is one of the more common "short" crank lengths.