Take off the left side panel. The metal plate you see inside the case is called the motherboard tray. There are a series of holes in this tray, some of which will match the holes on your board (there should be 9, ignore the ones that don't match up, those are for other kinds of boards). Screw the brass standoffs into these holes, then use the holes in the board to screw it down into the standoffs. YOU MUST USE ALL STANDOFFS! And don't make the screws overly tight. As long as they can't shake loose, they're fine. They just need to stay in place to give the board something secure to hang from. If you go too tight, you could warp and snap the board.
Then take the cables from your power supply, and connect the 24-pin mainboard connector at the front of the board, and the 8-pin CPU connector (should be near the top of the CPU socket)