1) Get a domain name. It doesn't need to be great, you'll be sick of hearing it whatever it is after a couple of years
2) get a shop website. These come in small medium and large. Small is Bigcartel, and a good start, medium is Shopify or EKM or your local equivilant. Either is a fine starting place and if you have a domain name you can always switch later. Make the shop pretty or at least neat
3) talk to indies and ask to buy books at wholesale. This is often slow and bespoke. Bigger indies can sell to you on concession (money later), which is a great way to get started
4) go to a business mail supply shop and get mailers that fit inside the various shipping categories of your country. Most books are a5 or close enough. Search for "book wrap" and maybe get some crate paper and a digital scale while you're there
5) take photos of the books and put them on your shop. Use a £20 lightbox and your phone, or take pictures on your garden. Just be neat and consistent
6) do your research on your national mail services rates. It's readily availible. Charge for the postage and add a little on top to cover pecking material, time, and you messing up how much it costs (you'll get better at this). You can reduce prices later
7) tell people you have a shop of cool curated indie books and they should buy them. Ask the people you bought books from to help out by telling their audience about you