At 15 amps, many of your larger power tools will trip the circuit, especially tools with heavy startup loads like compressors and radial arm saws.
Run more than one circuit, consider alternating outlets on the circuits (circuit a, b, a, b) and spread your tools out in a way that you are not likely to exceed 20 amps by using more than one tool at a time on a single circuit.
Consider contacting your local municipality for information regarding specific codes for Ground Fault Interruption, etc.
Lastly, PLEASE do not "double-tap" any of your existing breakers. Every new circuit needs to have its own breaker installed in your panel. If you are out of room in your panel, you can purchase "half-width" breakers that are essentially 2 breakers in one slot for some of your lighting circuits, which will free up room for a couple of 20s. Be very careful playing in your box, and always turn off and lock down the mains when you do work on your panel, and circuit.
For more information about wiring, hit ctrl-f when you are on the main board, and type "wiring" into the search box.
Have fun and be safe! Don't do this type of work alone.