Something that can be important to know:
a) On an old bike that hasn't been serviced in a while pedals can be stuck really tight
b) pedals come in "left" and "right", and with different threads. If you don't know quite what you're doing it can be difficult to separate between trying to unscrew in the wrong direction and trying to unscrew something that's badly stuck. A neat trick I've read somewhere is that the tool should be turned towards the rear of the bike when unscrewing a pedal regardless of side of bike.