Are you wanting to be a sole trader or a it's company? You need to know what your tax responaibilites are etc so you need to plan your business structure first. Sole trader is the simplest way to do it and is what most of these types if businesses are.
Open up a separate current account from your everyday and savings accounts etc. You want all your business payments to go out of and come into one account, with no personal payments in between so they are easier to keep track of.
You want your business name and mission statement. Think about your brand; what are you going to five customers that others won't. Pay for a designer to make you a logo, or do it yourself (use Canva - just Google it. Canva is a great platform to do all your own marketing designing on. You can use the free one or pay for a fee extra features). Once you have your logo, I'd suggest getting a load if sticker printed up (internationalstickers.co.uk are cheap, fast and good). Then, when you sell the clothes, you can wrap them in tissue paper and deal the paper with your own branded stickers. If you want to be super fancy, you could get cheap card boxes to put the clothes in and pop your sticker on the lid. You want to reinforce your brand with every sale.
Set up your Facebook and Instagram. Start posting. Do giveaway competitions, or 'win a £10 voucher" competitions and ask people to like your page and share your post to be entered into the competition. I usually ask them to comment so you get a list of all the people commenting snd choose a winner from there. Run them for a week, then choose a winner.
If you want to make your own website, I can recommend Weebly as an e-commerce hosting site. It's easy to use and design (just drag and drop) and they deal with the online store stuff. Just set up PayPal and you're ready to go.
You also need to read the consumer rights act - you need to know the law and what your responaibilites are for returns etc.
There's lots more, but get all that done - your brand, your plan, your selling platform and a customer base and then you're good to go.