You can start for very little indeed. Here is the minimum you need:
A domain name. If your target market is UK, anything which ends in .co.uk can be picked up for around £9.95 + VAT = £11.69 (that's part of my other business but I mustn't plug it here
)
Some space on a web server - there are many cheap, or even free, web hosting services out there. Google is your friend. That's another part of my business, but you can get far cheaper. I offer a more personalised service for hosting and design.
Some shop-type software. OSCommerce and Zen Cart are both superb and, as it happens, both free to download and use, including for commercial purposes. Your web host needs to supply servers with PHP and MySQL enabled to use either of these.
Payment gateway/merchant account. You can start off with Paypal and/or Google Pay - both free to set up and their commission is deducted before they pass payment on to you.
Some stock. Why not start off with "virtual" stock? Again, Google is your friend. There is a section on these boards about dropshipping and if you do a google on dropshipping, you will find many, many more sources of information.
So off you go. You can start off modestly and learn, whilst putting the money together for something more ambitious.
If this helps, please keep us informed on how you get on.
Good luck with it.
Mike