You have just beat Roxanne, the first gym leader and have received your first gym badge. That’s great! Now we have to go to Dewford Town. You need to find Mr. Brinney first, so he can use his ship and take you there. Ddewford Town is an island. Once you arrive you will have to go into the cave and deliver Devon Goods to Steven. You will receive HM Flash, which will be needed later on. Once you get to the end of the cave and meet Steven you should get TM Steel Wing from him, if I remember right. Then you can go to the gym. This gym is hard, from personal experience I had to skip it and defeat the third gym before I was able to defeat the second. When you enter the gym you will not see anything. It is pitch black in there.
To make it get lighter you have to fight a trainer in the gym, when you beat the first one the light will get a bit brighter, and so on. Once you beat the last trainer the gym is completely lit up. And you should be able to go to the leader, Brawly. This is a fighting type gym, I believe it is the earliest fighting type gym ever throughout Kanto, Johto, Sinnoh, and Unova. Anyway talk to Brawly, and you will get into the gym battle.
The first Pokemon Brawly will use against you is a Machop. It is at level sixteen, it should be fairly easy to take down, just be careful, if you got a Tailow on an early route it would be easier to defeat. Flying has an advantage over fighting types. And also I believe psychic types also have a type advantage over fighting types. So a Ralts would also help you out. The main worry is not to let the Machop use Bulk Up, if used too many times it will be terribly hard to take the Machop down. Once defeated he will send out a Meditite. The main worry here is not to let the Meditite use Focus Punch, it is a very powerful attack and if it hits you will be in a bad position. The Meditite is at level sixteen, same level as the Machop but is a touch more powerful, it is also part psychic type, to get a super effective hit you will need to use a flying type most likely.
Once taken down the final Pokemon he will use is a Makuhita at level nineteen. A bit high leveled, it also has a Sitrus Berry so its health will be restored if it gets too low. If you beat it you will get TM Bulk Up and the Knuckle Badge. Now to head to the third gym!