Lots of good advice above...
Paul Kirks book is good. You can learn while building rubs.
First, salt and pepper.
Second, add garlic powder.
Third, add onion powder.
There are a lot of recipes in it.
You can start with a balanced recipe and tweak.
The butt glitter recipe works well. You might like something a little hotter, you can tweak it. Or something sweeter...tweak it. Like oregano, add some.
Another way is find a rub that you like and try to make something similar.
I started making rubs first. In hindsight i wish that i started with a few commercial rubs first and then had a target in mind. It would have saved me some money. I am still experimenting. Enjoy the ride.