Once you have passed the first seven trainers in Route 24, you'll get to make the acquaintance of nine more in Route 25. The only thing you need to watch out for is the guy guarding the entrance to a ...

Even if you don't want to get any of the above items yet, do go back to Cerulean City (go through Mt. Moon again to get there) and enter the bike shop. Now that you have the bike voucher, you will ...