I second the above, but if you're going to have a winter baby if also buy a pram suit just after they are born so you know the size you need.
Not sure if you have a pep&co near you but they have decent and cheap baby clothes, the design isn't always great but occasionally you'll get some nice stuff in there x

Mothercare, h&m, primark, asda george, next.
You will need
hats - at least 4
hand mitts -at least 4 pairs
socks -at least 5 pairs
Bodysuits - 2 packs for each size
Sleepsuits - 3 packs for each size
You will need to prepare by buying the first 2 sizes you need which would be newborn and 0-3. I wouldn't buy too many outfits because they're still too small and sleepsuits are more comfy for them xx