http://www.wilton.com/technique/1M-Swirl
This is a great demonstration for cupcake icing, they also have a recipe which I prefer to the normal buttercream, they use half butter and half white fat (I use trex) it gives it a gloriously silky consustency and is really easy to work with. I used it for a cupcake birthday tower recently and got rave reviews
I'd keep decoration simple, either some pretty sweets, chocolates or maybe some silver balls. Sainsburys does some ready made sugar icing decorations if you fancy.
I've never made a huge one so don't have any recipes.
Have fun, I've only gotten into cake making/decorating in last year and I love it, I make celebration cakes for my family and friends