Depending on what type of robot your building and what you want it to do you will probably want some movement, the best way I know of is dc motors,stepper motors, geared motors, solenoids and servos. If you are new to motors here is a awesome instructables
.Motors are pretty much everywhere. If you have a old cd player, old robot toy or a RC car you'll probably find some dc motors but servos are less likely to be around you, I find, without buying some online or at a shop. I wanted stifiam to have two moving arms, some sort of mobility on the ground with wheel, a moving neck.