First thing is, you need to add up the amperage of each of what you have plugged in. You should be able to find that somewhere on the device. You might also be able to find the specs online. Add up the amps being pulled by each device. A regular household circuit usually has a 15 amp circuit breaker on it, so you can't run more than 15 amps worth of "stuff" on a circuit. Remember that there might be other things plugged into that circuit as well through other outlets.
If you're going to run a 100' extension cord, try to use one with "heavier" wires in it, say, 12 gauge rather than 14. It should be ok either way, but longer cords generate more resistance, and if you're running close to the max, that could trip the breaker. More resistance also means that you could potentially feel the extension run a little warm if you're pulling a lot of "juice" through it.