You don't have to be good but it helps.
I'd be ordering soon, because you'll need to put some paint on it before the event.
http://www.shadowforge.com.auin their characters have some female Blood Bowl teams, they are a local (in Sydney) manufacturer of miniatures which support this competition and would happily send you miniatures much faster than GWUK will. [/shameless_plug]
As long as you know the basic rules and are happy to lose a lot of games, you'll be fine. Swiss style tournament means that losers play other losers and winners play winners so you quickly reach players of your level.
If you want to ask further questions - go right ahead.