There are several types of firearms, but for the purposes of introduction they boil down to Rifles, Handguns, and Shotguns. In the United States ownership, possession, and usage of firearms is regulated by various Federal, State, and Local agencies. While I may touch on these restrictions you should check the laws and regulations that apply to you, because I will not go very deep into them. For the most part if you can buy something you can possess it, but that isn’t always true. You can legally buy some things in one state, but not take them to others. You can also buy individual parts of a firearm that are legal separate from each other, but can be highly illegal when combined together. This shouldn’t scare you off from the sport, but it should lead you to double check what is permitted, especially when ordering online.