So I used to be SUPER nerdy about planning this. I made a spreadsheet for ATIS/AWOS, Ground, Tower, Departure, and center frequencies for every cross-country flight. While that is nice and helpful, I will tell you that (like my flight tonight) your anticipated frequencies are not always the frequencies you will get. For instance, my anticipated frequencies en route KIPT-KPIT were: NY Center 124.9, NY Center 134.8, Cleveland Center 126.725, KPIT App 124.15. However, I never got Cleveland Center, but was instead handed off to Johnstown approach before being given a different frequency than anticipated for KPIT App. So, sometimes, it can be frustrating to have the wrong frequency programmed and have to redo it rather than just getting it straight from the horse's mouth. On the other hand, especially as a low time pilot, sometimes it's nice to know what numbers you can expect to hear, just in case you weren't sure what the controller said. If you look down and it matches what you planned, you can be relatively well assured of hearing it correctly.
So how do I find those frequencies? I start with my departure and destination airports. If you click on the airport in Foreflight, you'll get a display like this. You'll notice that you have your weather, Ground, and tower frequencies and then you'll be able to see the local approach/departure frequencies, which is who you will start FF with. If you DON'T see an approach or departure (like the example below), you'll see the local Center control. That's who you'll start up with if there's no app/dep.
View attachment 102735
Once you have your departure and destination airports planned like that (ie. Origin weather, Origin ground, Origin tower, Origin Departure/approach or Origin Center, then Destination Center, Destination Approach, Destination Weather, Destination Tower, Destination Ground) all you have to do is fill in the blanks in between. For most flights inside of 2 hours, you won't have many other frequencies, but if you do, you can find them in foreflight or on your paper charts. They are in nice little boxes for you:
View attachment 102737
You'll also notice that in the screen shot here, it denotes the crossover boundary between New York Center and Cleveland Center. Somewhere in that general region is where you'll get handed off. If a controller is busy, it might be before or after that line by a little bit, or they may hand you off to an intermediate (like they did to me tonight with Johnstown App), so it's only a point of reference, but it can be a helpful point of reference, especially in helping a lower time pilot stay ahead of the airplane. Once you do this for a while, you'll get the hang of it and won't need it as much. I don't do this anymore, but I often reference it before my flight, just to have an idea of where I'm going to be going and to whom I'll be talking.
I HOPE that answered your question, and I also hope that I was clear enough. If you have more questions than before I answered, I'm sorry! Feel free to ask and I'll try to help clarify what I can.