Don't panic! I'm 35 weeks and we still don't have everything lol
We also waited til after the 20 week scan and once we got the ok for that I went straight to the shops and let myself buy one little outfit
I found this website useful - they have checklists. One for essentials:
https://www.madeformums.com/downloads/MFM-WhatToBuyessentials.pdf
One for things that are useful but not everyone will want:
https://www.madeformums.com/downloads/MFM-WhatToBuyuseful.pdf
And one for things that aren't essential:
https://www.madeformums.com/downloads/MFM-WhatToBuynice.pdf
TBH I personally think there are things on all 3 lists that for me would be essential, but it's down to personal choice.
I would start buying small too as Loobylola says. Re the big things like cots, car seats etc they can be expensive so it's worth maybe putting aside some cash each month and buying these things later on in your pregnancy. This also means your house won't be bursting with bulky items and getting in the way! You can get some good deals in sales though, so worth signing up to things like Mothercare newsletters and jumping on items when they're on sale. If you join the Boots parenting club you get vouchers for baby items and extra points on your boots card
If you're not fussy about 2nd hand it's definitely worth a browse at car boots, and in my area there is a 2nd hand kids group on Facebook. We've only spent £30 on clothes so far and we are all set for 0-3 months we got so much and in great condition too, because really babies don't spend long in their clothes.
Congrats on your scan!