To purchase a firearm legally you need to first check the laws in your state about owning a firearm. Find a reputable firearm dealer and fill out all the needed paper work. There is a waiting period before you can purchase some firearms. You will have to have a background check.
Go to your gun dealer in your state, he will explain it. After all, he wants to sell you the weapon (personal protection). In a free country you shud not be required to explain why you want the firearm. You have every right to own it, "your rights shall not be enfringed".