Buy one or two of the nappies you like to see how you get on with them. Most nappies fit most babies quite well, although some of the less well-known brands have an unreliable fit. Tots bots, motherease, fuzzi bunz, bambeasy, weenotions, and upsy daisy nappies are all relatively safe bets!
Its also good to "pace" yourself as you dont want to buy loads of nappies then see lots of other pretty ones that you want! Well, you might end up doing that anyway