To get to this world you need to beat the boss of World 5-Castle as Mini Mario.
World 7-1
Start off by heading right and riding on the moving platforms. Let the second platform fall and head left. Jump to grab the first Star Coin. Ride the platform back to the right and jump off before it falls. For the second Star Coin, you must hurry. After the platforms, there are two ?-blocks and then a hanging wall. To the right of that is flying ?-block. Jump on it and let it carry you to the right. At the end, get off of it onto a cloud. Head up through the green warp pipe. Swing yourself between the moving poles to reach the second Star Coin. Head down the green warp pipe on the right. Unfortunately, this coin made you skip the checkpoint, so you cannot die. Go right and up the green warp pipe. Hit the flagpole to finish the level. For the third Star Coin, you must do the level again. Go back to where you jumped onto the flying ?-block, but don't get on it. Keep going right and through the green warp pipe at the end. Head right and get on the top moving platform. Grab the third Star Coin, and finish the level like your first time.
World 7-Ghost House
When you start off, hit the first block closest to you to reveal the switch. Jumping on the switch will reveal 5 doors. Head to the door on the left to get the first Star Coin. Exit the room and got to the door on the top of the screen. In this room, butt-smash (jump + D-pad down) your way to the second Star Coin. Activate the switch to the right of the Star Coin and leave the room. Go to the door on the bottom right. There are hands in the sky which are pointing towards an invisible box. Jump up to reveal it and then activate the switch. Once activated, head right to the moving platforms and climb to the top to enter into the door. To get the third Star Coin, activate the P switch and dash right as fast as you can. As you move to the right, you'll notice that some of the blocks are increasing in height. Climb up to those blocks, head right until you reach the Star Coin. Then head left until you fall onto the lower platform and move to the right and enter the door to finish the level.