I’m 17 and been on console for about 7 years and i want to switch to pc. I have about £1000-£1250 budget I already have mouse, monitor and huge mouse pad already

I want to have a pc within the next 2/3 months I am aware that some pc parts such as gpu are inflated due to corona thought I’d just note that. I have a basic knowledge of pc parts and Ik what most of them do as I roughly paid attention in my computer science class...(7 GCSEs at the same time was hard)the thing I’m struggling with is how I know everything is compatible(I know that i need a atx case if I have a atx motherboard)was. Was looking for any advice. Was also wondering if anyone has experience with switching from ram to M.2 and if you think it was worth the upgrade