What you'll have to do next is head to Route 12, on this route you'll need to have a Pokemon with Surf. If you don't have a Pokemon compatible with this HM then you can trade with the breeder guy who is willing to give you a Lapras. From Route 12 you'll come across Courmarine City, (take the monorail) which is where you'll gain your Plant Badge from Ramos. He specializes in Grass type Pokemon.
Hope I've Helped!