Don't worry I've figured it out. You trade it onto firered/leafgreen and teach it via someone in silph co. building.
You need to give staryu a water stone.
Yes the only way you can get a starmie is by usinga waterstone on a staryu.
You have to get a super rod from a fisherman and check your pokedex to see where staryu is and catch a staryu in that area with the super rod. Get a waterstone and evolve staryu into a starmie.
You catch a staryu at lilycove by fishing at the shore, (takes a while, use super rod) then use a water stone on the staryu to evolve it into a starmie. (Starmies are tite)
You can exchange them for water shards at HUNTERS HOUSE near mossdeep city. You can give your water stone to staryu and it will evolve into Starmie! NOTE: THIS IS FOR POKEMON RUBY! -Sammi out! :P
yes staryu is rare in Sapphire and ruby
You catch staryu in Mossdeep city right behind Stevens house.
Super rod.
You can't catch staryu in firered only in leafgreen or ruby or sapphire or emerald.
No, you cannot. You have to trade Starmie over to your FireRed from either Ruby, Sapphire, Emerald, LeafGreen, or Pokémon XD: Gale of Darkness.
Fish with good rod in lilycove city.
No, it can only be found on emerald or ruby and Sapphire