cerluen cave
Go through Mt Moon and there will be a guy in there who will give you the choice of either a dome fossil or a helix fossil. I hope this helps.
You Cant
Helix
it evolves into a kabuto
The Dome Fossil can be revived into the pokemon kabuto.
Fossils cannot be created in Pokemon FireRed, there is a finite number of them to find.
The dome fossil is only in firered and leafgreen, you can get the claw fossil or some other fossil, the claw fossil is anorith the other one is lileep.
The Left One
Take it to the museum in cinnabar city to have it revived
cinnabar island research center
If taken to cinnbar island's research lab you can revive the Pokemon inside the fossil which will be Kabuto.
You need to battle a guy and when you kill him you get the fossil.