How can I get the star on the roof on big boos level and how do I get ..

You need to wall kick your way up there. It takes time and patience.

First off is Luigi. Go in as Mario or Yoshi with a Mario cap to the Big Boo level. ( If you didn't get the first star, do that first.) Go on the second floor and go to the door on the far right. Hit the ? block as Mario and become Mr. Ballon. Float on to the platform above. Ground pound to become normal again. Open the door and look to the right. Theres a Luigi painting! Jump in.

You'll wind up in the merry go round. You look around and see 4 doors. There is a series of rooms, but only one door will take you to the next room.Take each door in the following order:
Room 1:Right
Room 2:Left
Room 3:Right
Room 4:Left
Room 5:Go across the bridge and take the platforms up. Jump into the hole.
Youll see a mirror in front of you. Run up and stand in front of it. Soon, Big Boo will appear. You have to hit him 3 times.
First time: He's invisible, so look for his shadow. You can also see him in the mirror.
Second time: You can see him normaly.
Third time: Same as the first.
Go to the room with the players doors in it, and open Luigis door. His special abbilities? The ? block makes him invisible AND invincible. If you dont jump into water he can run on for a few second. When he backflips, he slowly twirls down. And he can do something similer to Yoshi when you hold the B button after jumping.
Now for the star on the roof, get to the room with Luigis picture the same way as before, but don't go in the painting. Instead, go up the steps and into the door. Fight Big Boo to get the star. Go to where the star is BUT FIRST, hit the ! block for a extra life.