What pedal fits my crank?
There are basically two common sizes for pedal threads.

Almost every adult bike will have 9/16" threads, with the LEFT foot pedal being left-hand thread (ie, backwards), and the RIGHT foot pedal being right-hand thread (ie normal)

Children's bikes have 1/2" threads.  Just about all of them are one left hand and one right hand thread, but its also possible for small bikes to have two right-hand threads.   This is workable because your average small child isn't putting out high wattages. 

There are some very rare cranks with 12mm or 14mm threads, but in reality you won't have one of them.   I'd be 99.9% sure its a 9/16" thread.

To be super-sure - take the old pedals with you when you go to buy your new ones.