There are some good books about that you could probably get from the library.
There are loads of websites about too so they might be worth a look for some ideas. These ones have some nice looking cakes that might give you some ideas.
http://www.cakesbyann.co.uk/
http://www.cooks-weddingcakes.com/we...pictures.shtml
http://www.thecakestore.com/weddingcakes.htm
Quite a few people get the cakes from M&S or one of the supermarkets and then decorate them. Might be worth doing that and apparently they are pretty nice.
My mum's friend made our cakes (we had 2) but she did a trial run of the main one. She just did one small tier and decorated it for us to get an idea of what it would be like. It might be a good idea to have a practice run first.