how do you get to star world??

I'll post where you can get to each of the Star Roads:

Star Road 1 - Find the secret exit to the Donut Secret Ghost House. Enter the room where you would normally exit, and find the Pow Block. Walk over to where the Pow Block reveals the secret exit. Jump up and stand outside the exit door, but don't go through, jump up instead. You should activate a piranha plant to move upwards. Climb it, and run to the edge of the balcony to where another door is (you have to be quick, though, because it will disappear when the Pow Block music stops). To beat the Big Boo in this room, pick up the blocks on the floor and throw them at him. Three hits kills him. You should now have unlocked the path to Star Road 1.

Star Road 2 - The entrance is in Vanilla Secret 1, but you need to have beaten the Blue Switch Palace to unlock it. Near the top of the level, grab the Springboard and jump over to the two Blue Blocks (still holding the Springboard). Drop the Springboard and jump on it to reach an otherwise inaccessible pipe. Go through this pipe and head to the goal at the end. You should now have unlocked the path to Star Road 2.

Star Road 3 - You unlock the path to Star Road 3 by beating the Soda Lake level. To unlock the path to the Soda Lake level, use your cape to fly under the Cheese Bridge goal, and keep going to find another goal behind that one. This is really hard as you have to fly a long way whilst avoiding chainsaws, but keep trying and you'll eventually get the timing right.

Star Road 4 - To unlock the path to Star Road 4, you have to beat the Forest Fortress. Halfway through Forest of Illusion 4, there's a floating blue pipe with a Lakitu inside (this is just after the halfpoint goal). Jump onto it, kill the Lakitu and enter the pipe. There's a key and keyhole inside, so you know what to do. Doing this unlocks the path to the Forest Secret Area, and beating the Forest Secret Area (which is really easy) unlocks the path to the Forest Fortress.

Star Road 5 - In Valley of Bowser 4, keep Yoshi throughout the level. If you lose him, start again, as you won't be able to grab the key required otherwise. Near the end of the level, there's a key that's surrounded by rock, so you can't get it normally. Use Yoshi to roll out his tongue, which goes through the rock and will grab the key. Then proceed to the keyhole. Sound easy? Well, it's actually quite hard, because going through the level without losing Yoshi is difficult, and to get the key, you have to stand on one of the stones sinking into lava, with a bunch of those Charging Chuck idiots kicking footballs at you. But yeah, this unlocks the path to Star Road 5.