Go up from slateport city and you will find a house up there. Go right from under the bridge and go up. Battle with your rival MAY and go up, left and up and you will reach Mauville City. Go to the Pokemon centre and just right to it is the Bike shop. Go in and talk to the person and he will give a bike. There are 2 types of bikes.
Mach Bike (the bike that goes very fast and is needed to go over the cracks) .
Acro Bike (the stunt bike slight slower than Mach bike and when you press B it jumps) .
Chat with our AI personalities
The Mach Bike.
Go to the bike store in Mauvile city
You Can Aquire Either An Acro Bike Or A Mach Bike In Mauville City At Rydel's Bike Shop.
In mauville city
get the acro bike by ridel and hold the B-button