In Diamond right after you have beaten the first Gym leader you will have to go north of Jublife City. At the exit of Jublife city you will encounter Prof. Rowan and Dawn. You will battle two grunts with her. After you fight the grunts you will go up a rode and battle a few trainers. You will run into a cave, enter it and use rock smash on one of the rocks to get out of the cave. You will then battle more trainers. After you battle the trainers you will be in Floarama. You will have to go to a a building. You must fight a grunt there. Once you've battled the grunt you will go to the far north-left of Floarama. There will be an entrance there. Go in and fight the grunts. They drop a key to the first building. Once you have defeated the first Galactic commander you will have to head north. You will battle a few trainers on the way to a forest. Enter the forest and you will encounter someone who wants to go threw the forest with you. After you passed the forest you should be at the second gym.