If you have a fare for crafts and are able to make your own jewellery or stitch beautiful dresses then you may consider selling them online. There are many places where you can sell items online such as eBay or Esty. These sites will allow you to list items for sale and for them to be shown on their website and possibly on others as adverts. You will need to consider how much each one costs to make in terms of materials and also your time. You need to be sure that you are selling them to make a profit and are not going to end up losing money.
You should also set up a business with HMRC as you will have to do a tax declaration, although this is not normally due for approximately 18 months (depending on when the business started). You need to make sure that you keep good business records of all your sales and your expenses and have receipts and invoices to back this up.
You may also need to take out insurance to cover you in a situation where someone gets hurt or becomes ill from using your product for example.