Go west from mauville city using surf to the other side of route 118 then head north to route 119 head for Fortree City.
If your not ready I would go back to all the places you could surf at (and other HM spots you haven't done yet)
Petalburg City
Surf north east of the gym to get a max revive, also surf south west to get a ether there is also a rare candy hidden east of the ether
Route 104-109
Surf and fight trainers for more experience
Route 110
The Trick House also get a rare candy north east of the trick house. You also have New Mauville town side quest to do by talking to Wattson after beating Norman
Route 111
Get the item by the winstrates
Route 115
The north side of the route battling a couple trainers and get items
The Abandoned Ship
Doing the first part of the ship before getting dive of battling trainers and getting ice beam
There are place you can go to after getting surf
Hope I helped