Now that you beat the Fighting Gym, go to Granite Cave. Now that you can use Flash, it is time to put it in use. The cave has only one real path, so just follow it. You should find a guy at the end of it. He will give you a few things, then leave. Now go back and talk to the old man. You should see 3 options, if so, select the second option, and he should bring you to a new area. If not, check around to see that you did not miss anything. Once you make it, explore the beach, and fight the trainers you see. Make your way north to Slateport City. There isn't much in that city, but its still large. If you go Northwest in this town, you will notice an odd red building. Go inside and talk to the little girl to get a Pokeblock Case! In the middle of the city there are many Team Aqua/Magma members. Go into the building below it and talk to everyone, and they should be gone. Now go inside the museum, and pay the Museum fee. Talk to the first Aqua/Magma Member, and he will give you something for some reason. Continue to explore, they wont cause any harm at all. Well not yet anyway. Go upstairs and talk to the guy there. 2 Aqua/Magma members will come upstairs and fight you one at a time. One has a level 15 Numel (Ground/Fire), and the other has a level 14 Zubat and level 14 Numel. A few moments later the Aqua/Magama leader will come up as well and talk to you, but he will quickly leave.
When you are done exploring the City, walk north. You will find a house you cannot pass, but just ignore that for now. Continue walking along and enter the next house you see. You will see something shine, so go there and talk to it. Some guy will appear, and he will open the door to the north, and enter. Fight the trainers you see and take the items. You will find him again, and he will give you something. Then you can leave from the other exit, but you cannot go back again. Be sure not to miss anything. When you leave the house, go left and fight the trainers you see. Then when you reach the dead-end walk back, and pass under the bridge. I recommend healing first, since the Pokemon here have a tendency to paralyze you. You will not be able to walk east anymore, and have to go up. You will find your rival there. Although he will have only 3 Pokemon, he is not a pushover. Once you beat him, continue along that path fighting whoever you come across. You will eventually reach Mauville City. Be sure to go to the right of the town, because inside one of the houses, a man will give you HM06. HM06 contains Rock Smash. Then go north and go to the bike shop to get a bike. You will need to talk to him a few times to get it, but its worth it.
In this city, there is an electric gym. However, the Gym Leader is tough, he has 3 Pokemon between level 20 and 23. I recommend going to the right to beat up a few trainers. Don't worry, its a dead end, so don't worry about getting lost. When you think you are ready for the Gym Leader, go back to town, heal, and then go to the Gym. You will see a few guys outside. You must fight one of them, when you do, they will leave and allow you to enter. The Gym itself is a fairly easy puzzle, but only when you fight all the trainers. To change the electric lines, step on the tile that looks like a volt image. And don't worry about touching the electricity, nothing happens. The Gym Leader is called Wattson, and is kinda hard, so be sure to bring a Ground type along. His Pokemon are Magnemite, Voltorb, and Magneton. He will love paralyzing you, and will do so often. Magnemite is not that tough, and is level 22. He will give 418 Exp when defeated. Voltorb, who is level 20 has Selfdestruct as one of its moves. I recommend using a non-ground type for Voltorb. He will dish out 441 Exp when beaten. The last in his arsenal is Magneton. He is level 23 and can be a pain. Just stick to a ground type for this, and you should do fine. He will give out 749 Exp. When you beat Wattson, he will give you TM 34. When you win, he tells you to go visit your mother. She will give you an item to increase your amount of money when you win a battle.
You finally got your 3rd badge! It will let you use TM06, Rock Smash. Now lets clear up Rusturf Tunnel! Go left, and fight all the trainers there. There is also another Double Team match over there, if you talk to those 2 girls standing together. Either way, pick up any items you find, and make your way across. You will eventually come to Fallarbor Town. There is nothing much there, as they do not have a Gym. Teach someone Rock Smash, then enter the Rusturf Tunnel (the green foggy place). Break the rocks, then talk to the karate type guy. He will give you HM04 for your work, which is Strength. You cannot use it yet, just hold onto it for now. Now go back to Fallarbor Town.