Uhm, for a pre-made layout, you go into a) the CSS tab or b) the HTML tab on the right side of your 'My Mibba' page. I can't remember which, sorry.
If you want colours, sizes link, youtube e.t.c., the url is:
www.mibba.com/forums/bbcode
Type that in and you'll find all you need to know.
I'm just going to give you a heads up about your spelling and stuff. I'm usually nice about it, but others aren't and will abuse you for not having good grammar/punctuation e.t.c. If you want your work edited to prevent that, then come talk to me.