First time buying a gun legally tips?

If you're over 18, you can probably shoot pistols at a shooting range. One that rents guns would be a good option- you will probably have to have someone 21 or older there to sign for the pistols, but that depends on the range.

Why am I suggesting this? Because the first thing you need to do is go take a firearms safety course that includes hands-on time at the range. Considering that you obviously don't know anything about guns (not an insult, simply an observation), you don't know what you want. I promise you that.

However, so long as you are over 18, you can buy any rifle or shotgun. After you take a firearms safety course, consider buying a .22LR rifle. .22LR is one of the least powerful firearms available, and it is also the cheapest to fire. One round of .22LR ammo costs about a nickel (yes, $0.05) while a round of 9mm can be anywhere from $0.25 on up. That means that you'll be able to practice shooting much more with the .22LR, since ammo is much cheaper.

There's another benefit to .22LR. It isn't as loud and has almost no recoil. That's good if you're a beginning shooter. You can focus on the proper procedure for shooting without the distractions of heavy recoil and loud noise... but you do still need hearing protection, even for a .22LR.

Now, one final word: a light gun will have more recoil. As a beginning shooter, you don't want a lightweight gun. The heavier the gun, the less recoil you will experience, given the same cartridge.

EDIT: Oh, and classes aren't usually necessary, depending on where you live. They're just a good idea. As for a permit, that will entirely depend on where you live. Some states require permits to purchase a gun, and some states don't.

Incidentally, a .22LR rifle, like I suggested, will be significantly less than $400. The most you'll pay for a basic one is $250, and you can get some for $150. That's yet another reason I suggest a .22LR- the gun is less expensive.