It's either a 15mm open end wrench on the spindle outside the crankarm (you may need a special extra thin pedal wrench if there's not a lot of space between the crankarm and the pedal body to fit a standard wrench), or an 8 mm Allen on the end of the spindle inside the crankarm.

Just remember that if your pedal takes the Allen wrench on the inside, you're actually pushing the spindle through the crank, so you have to turn your wrench the opposite direction you would normally turn to loosen something.

Also remember that the right pedal is right-hand threaded and the left pedal is left-hand threaded.

You might possibly also need extra leverage if the pedals are very tight - a long breaker bar if you're using an Allen socket, a length of pipe over the end of your wrench if it's an open end wrench or hex key ... maybe even a soft hammer to the end of the pipe. Just, before you start waling on it, make absolutely sure you're trying to turn it the correct direction. I always have to think about it, no matter how many times I do it.

When you re-install the pedals, put a little bit of anti-seize lubricant on the spindles (the same stuff you put on spark plugs).