Anyone who's not a prohibited person (people convicted of felonies, basically) can buy a firearm. Some places require you to pass a course in order to get a concealed carry permit, I think. Many states have laws which lower the purchasing age based on whether or not you've got a safety course under your belt. Where I live, passing a hunter safety course lets you buy rifles, shotguns and ammunition under 18 if you've got a hunter firearms education cert. I definitely used to take advantage of that.

I'd look up the laws based on where you live, but you're probably able to buy whatever you want as long as you're 21 or older. A hack is to see if your local shooting range offers instruction. One session will go a long way. Online can be useful too, but there won't be someone to correct you or answer questions. Answering questions and being hands-on to correct someone were what I was doing the most when I taught firearm safety.

It makes me happy that you're interested! Every American citizen should know how to properly use firearms.