Only NFA tax stamped firearms (machine guns, short barreled rifles/shotguns, destructive devices, etc.).
To give you a short answer- you don't. Like MOST US States, there is no requirement or provision for you to register an ordinary rifle, shotgun or handgun with anyone. If you buy a gun from a dealer, the dealer will make a record of the purchase (a form 4473) which is retained at the dealership.
You would have to go to your local gun shop and register your firearm there. Then, after completing that, you would have to go the gun range and register yourself there, with your gun permit.
Depends on where you are. In MOST of the US, there is no requirement to register an ordinary firearm. There is also no place to register a gun- on line or otherwise.
Depends on what state you are in. In MOST of the US, there IS no requirement to register a gun.
Texas does not require gun registration.
There is no requirement to register guns in Florida- or in most of the US.