If the old ones are stuck spray liberally with WD40 and leave overnight for it to have a chance to work.
Sylv's tips are good, and if you have a proper pedal spanner (and the pedals you have accept one which some current pedal models stupidly don't only giving you the allen key/grating your knuckles on the chainring option
) then you should have enough leverage.
When undoing, so pulling the spanner towards you if you are behind the bike with it upright, it's easier with tough ones to have the spanner at 9 o'clock so you can use your body weight on it. When doing up it's a doddle as once you start the thread just slip the spanner on and pedal backwards using the spanner and it will tighten itself quickly. No need to over tighten either, just till it stops and then pinch it off as when you pedal you are turning against the thread.