help with tax so confused

So i started in may last year just read on here that i need to keep all reciepts for set up cost etc...
I have kept a paper typed reciepts of how much i have earned from childminding ive got my medical reciept needed before i could start childminding i brought a pirateship play equipment for the garden plus a few other reciepts but dont think ive got every single reciept of everything i brought had days out last year with mindee didnt keep any of those really confused and when am i meant to do a self assesment form???so i can prove how much ive earned just not some things ive brought i brought a car seat from ebay should i try and get a receipt for that?did go out and buy a buggy and the lady in the shop wrote it down for me but wasnt on official paper as it is a charity shop. Really dont know as has its my first year just not sure if im doing it right.
Please any advice much appreciated:)

if l buy from ebay l print l print off the pay pal reciept, when l buy from charity shops l just write on the top of the till receipt what it was l brought.

Anything less than a tender doesn't need a receipt. The exception being food when you need every receipt if there was ever a food poisoning issue