I'd definitely recommend hitting garage sales now and looking for baby clothes. I generally get 4-5 pieces of clothing for $1, so you can build a good wardrobe quickly & cheaply even if you're picky (I am--no stains and 100% cotton). It's harder to find unisex clothes, but you can either buy some for each gender, or just look for unisex and buy more once you're pregnant and find out the gender or the baby's born, depending on the time of year.
I'd recommend researching the different types of cloth diapers and figuring out what sounds best to you. Prefolds & covers are definitely cheapest, but more of a hassle. And it is possible to buy used fitted diapers for not much more than prefolds, especially in the newborn size. As PPs have said,
www.diaperswappers.com is a great place to get your diapers! Also, I personally wouldn't buy more than newborn and maybe small size diapers at first unless you're only buying prefolds and covers. There's a fair chance you won't like whatever diapers you start with, and if that happens, IMO it's a lot easier to find some new (used) diapers to try for the next size than to sell & then replace S, M, & L sizes of diapers if you've stocked up for all the way through. Although if you get a great deal on something it might be worth buying bigger sizes
