If you are starting from your home airfield it's easy. Note down the likely frequencies on your plog, in the order you expect to use them. Talk to the people you normally talk to on departure. At some point they will suggest you change to "X" and usually give you the frequency while they are at it. Read it back and do so. The only reasons you have written them down are A: saves you doing it in the air and B: in case you lose radio coverage with your first service provider. It gives you a good guess at the next person to call. Alternatively, go NORDO. Though probably not where you fly :-)
Returning home, after clearing from the airfield, reverse the procedure by trying the last frequency used. There, how easy is that? BTW, instructors don't usually mind you asking this sort of question, even if you aren't a student any more. Buy them a cup of tea to drink while you chat.
Enjoy spreading your wings!