You're situation is similar to mine -- I only recently started baking from scratch (after box mixes for family and friends for 10+ years) and I love it! Definitely learn the science behind why/how baking works. You can start at the library (search for science of baking type titles).
As for developing your own recipes, that is a lofty goal which you might be ready for after a lot of experience, but there's absolutely nothing wrong with using another's recipe (even a common one) and either making a few tweaks of your own or some tweaks recommended by the fabulous bakers here on CC. Baking is really an art and a science and there is plenty of skill required to bake a recipe properly, so don't feel it has to be a recipe you made up yourself to be a worthy accomplishment. If you find a great existing recipe (which will take plenty of trial and error in itself), take pride in executing it flawlessly and enjoy the fabulous results. I really don't think anyone will think less of your cake if you didn't write the recipe yourself!
Enjoy the wonderful world of scratch baking, I'm sure it will be a rewarding adventure!