When you enter the castle, you go straight up the stairs. There's a locked door right there. How do you get in there? Where's the key?

You have to defeat the second Bowser in the basement, but you can only reach the basement by defeating the first Bowser through that big star door.

Plus, when you do get to him, run away from him in the begenning, but when it kinda slows down, grab him and throw him, (NOTE: When you get there, you'll know what I mean)

Kay! Thanks =D

Go behind him and press A to grab his tail. Spin the control stick to and press A again to throw him
(Thats if you don't know how, its quite easy anyhow)

I forgot, you need 30 stars to get there