ESCs (Electronic Speed Controller) are pretty straightforward- there are two main types, either brushed or brushless (easy to tell the difference, brushless motors have three wires, brushed, only two). Other than that, there is an amp rating. Examples: 40A brushless ESC, 18A brushed ESC, etc.

If you are starting out, and you don't have a Wattmeter, stick to the manufacturers recommendation for motor, ESC, and prop.
Andy