Welcome to the forums
First if you haven't done so go take a training class or two.
Learning safe handling, storage and usage is very important.
From there what you but will be influenced on your needs, budgets, location etc.
Some places place severe (unconstitutional) restrictions on what you can but, own, carry etc. so you need to be aware of them even though some of us feel they're totally illegal and should be overturned.
That said, the triad of a pistol, shotgun and rifle are the usual basis for firearms.
If you haven't shot a pistol before a full size 9mm is going to be much easier to handle then say a compact or a compact 45. Don't do what some of us did and start with a gun that's better for concealing then learning good technique.
A shotgun is flexible and something I think every American should own and know how to use. The vast range of ammo they can spit out is amazing. The damage they can do is brutal and there's just something cool about racking a pump action that I think should be part of your life experience.
Rifles come in all sizes, shapes and formats. I'm a big fan of ARs or what is often called the modern sporting rifle. A bare bones AR can be had for about $400 and they shoot cheap 223/556 ammo. They're often called Adult Legos as you can easily customize them, modify them and make them exactly what you want and/or need. Plus they're just a lot of fun. I'm currently building one from parts, after having bought one, snapped an upper and lower together, and now taken the leap to know them from the insides out.
Of corpse there are other types of rifles such as AKs, mini 14s, bolt actions etc. but I'll leave them to someone that knows them better than me.
Oh, and know there are tons of deals out there so don't rush buying anything.
One final thought. Reloading or more properly hand loading ammo is also a lot of fun. It's also a great way to get high quality ammo fairly cheap and really understand your firearm and what you can do. I mention this not to encourage you to look into it now but to think about it in the future if you get serious about your firearms.