Talk to the dojo master and ask to train. There are two mini games to play. You can buy better training dummies for skill training, or the brick/silver/gold bar for strength training. I think some are from the fishing guy and some are found in the excavation mini game.
Dojo training doesn't give you any money. Visit the construction site and do some labor for him. After a while he'll invite you to do an excavation mini game for more money. You can get better drills from the shop and the fishing mini game. Once you can buy the camera, you can film your sex tapes and sell it to the black market at the construction site after midnight.