Although we as tiers often complicate our flies with the various materials we have available, simple flies are often the most productive as far as catching fish. Doesn't usually matter what species we chase either.
There are literally thousands upon thousands of fly patterns you could learn to tie. At your age I'm betting your resources are limited so stick to patterns that fit your area. Since you've mentioned both trout & bass, there are patterns that can be used for both, primarily by changing hook sizes. Or, there are patterns that can be tied with the same materials for both.
For example, an Elk or Deer Hair Caddis dry fly is a very good trout fly & can be a good bass fly, especially in streams. However, by learning to spin the elk or deer you can make bass bugs or other large dry flies with the same material.
That would allow you to limit the supplies you need & still tie different types of flies & learn additional techniques.
Check out some of the online tutorials such as Liphookedau mentioned or those by Charlie Craven. Go online & browse patterns (Google Images is great) & see if you can find some you would like to tie & use. Pick a few & obtain your materials & go from there. Once you get good at a few patterns, then expand to additional patterns as you can afford to get materials.
Also, and you may already be doing it, don't be afraid to experiment. Just keep it simple & you'll have plenty of flies to fish with & learn as you progress.
When I look for simple patterns, no matter what type I generally search for patterns that use no more than 3 materials. There are many!