You need to download a file of a skin. I suggest going to google and typing in skindex, and clicking the first link. Once you find a skin that you like. You shall click it and on the right there will say download and upload skin. You can 1. upload it and it will bring you to minecraft.net to change it or 2. Download a file of the skin. When you are turning minecraft on (the premium mc which you download from minecraft.net) you click skins, and upload the file with it. You click either slim or classic, save and then you play and you will have a skin. (!) The skin might not be on your account straight away as it takes time for it to upload to your profile in game. The average time is 5-60 minutes but it can take more. You also said that you have changed your skin but maybe you didn't click save or you changed it recently so it will take some time to be uploaded. I hope that helps if not then I can try to help you on discord if you have it.