Soon after defeating Brock, your Pokemon journey will take you deep into Mt. Moon where you will be given the choice between two different fossils. This event is story-based and impossible to miss, so if you haven’t seen it yet, don’t worry.
Both pokemon are Rock and Water-types, so ultimately the fossil you pick is only dictated by preference.
The Helix fossil can be resurrected into Omanyte and subsequently, it’s evolution Omastar.
The Dome fossil can be resurrected into Kabuto and subsequently, it’s evolution Kabutops.
Well, you’ve got the rocks, now what?
It’s going to take a while from where you retrieved the fossils themselves, but upon reaching Cinnabar island, you gain access to the Pokemon Lab next to the Pokemon Center.
Head straight to the back of the lab and speak with the man next to the funky-looking machine, and he will offer to take your fossil and resurrect it back into its respective Pokemon.