Hi Luvapug
its completely up to you which type of show you enter first.Open shows are smaller but you may get a less reliable idea of how good your dog is since you may have an all rounder as a judge and there will be less pugs entered.
A champ show is more expensive and much more 'formal' but you will get to meet more pugs and hopefully get a better idea of the quality of your baby.
My first show was at a champ show...and I had a great time!
As for shows...have a look at
http://www.prodogs.co.uk you will find a list of champ and open shows.
Archer