I did lots of reading initially and then started buying the odd one or two nappies - various systems, fabrics, designs, and manufacturers. It would be good if you can get to see some nappies in the flesh - I was surprised just how bulky some of the designs were and how sleek others were. One thing I'm very glad about is that I didn't run out & spend £££ on a whole system, before the baby came along. From 0 to 6 months, I've muddled along with a mottly assortment of nappies & wraps, mostly from ebay or from the bargain section of the various nappy websites.
Don't feel you have to be utterly faithful to one manfacturer - mix & match is fine IME - e.g. Motherease wraps with TotsBots nappies etc.
Consider the other people in your baby's life - if truth be told, my DP needs a re-usable that's as close to a disposable as poss. to keep him from bending my ear & swearing too much at change time.
Learn about the various fabrics and their associated properties (so hemp, bamboo, cotton, microfibre etc.) - each has their advantages & disadvantages wrt to laundering and absorbancy. Think about how you want your system to work - how will you dry your nappies for instance - can you line dry or are you going to need to dry indoors?
It might sound daft but maybe have a look at a growth chart - months 0 to 6 babies grow pretty fast and then slow down a bit. For me this has meant I could justify paying more for a medium nappy since DD is going to wear it longer (on her current weight centile, the medium Itti Bitti will last until she is 2 years old in theory!). My first size (birth to 5 or 6 months) nappies were much more cheap & cheerful than what I'm using now. If you know you're having a baby girl, then frocks & tights tend to be a bit more forgiving on a real nappy than trousers are.
If you can cram in a bit of research in the next few days, Real Nappy Week 2010 starts (I think) 26th April, so many websites do special deals and freebies around this time as promotions - it would be good for your purse if you can take advantage.
Also beware as this real nappy business is very, very addictive. Just when you think you've found your Holy Grail of nappy combo, something else comes along and you'll start lusting after something new
Also consider reusable wipes along with your nappies.
I have in my first size collection Little Lambs, Lollipop, Ella's House, Motherease Sandys & TotsBots first size nappies, with Motherease wraps, plus some very teeny unbranded nappies & some prefolds & muslins for newborns. I probably spent about £50 in total for that lot. Now DD is 8 months & I'm on the Itti Bitti SIO with 3 BumGenius onesize thrown into the mix (they were on a 3 for 2 offer and therefore too good to miss). Utterly loving the Itti Bitti's TBH - very slim fitting and as easy as a disposable to use.