Im stuck on Diamond and i have only recently started the game but i don't know ho to get a bike because i have defeated the first gym leader but i cant get over the rocks to the next city where do i get a bike and how do i get to the next city?

You have to save the bike shop owner from Team Galactic in Eterna City, go there and it will flow.

but i don't know how to get to Eterna City!

Which city are you currently in?

the city with the mine in it(sorry i don't know the name)

Head back to Jubilife (the one with the TV station) if you have beaten the gym leader and be ready for a battle with Team Galactic then. :-)

who?

The big evil team.

hahaha lmao

I can't get Arceus, how should I do?

I'm already send Manaphy egg from Pokemon Ranger, but I can't find my egg! What's wrong?

To get Arceus, who have to get the Azure Flute through nintendo events. Then, go to the Spear Pillar. You will be asked if you want to play the flute. Say yes, and stairs will appear. Go up the stairs, and you'll find Arceus.
As for the egg, it may be in pal park, because that is where all transfered pokemon go. Also, try checking pokemon ranger again to see if it went through correctly.

I can't go to Pal Park too, or maybe I must go with S.S Sinnoh?

Go south from Sandgem, and surf down there.