First, you'll want to find a skin. There are lots of skin websites out there with awesome skins. You'll then want to click a "download" button if there is one. You might want a parent to help you. Then, search up "Mojang.com/login" and you login. When you're on the Minecraft page, click Profile at the top. Sign in and everything, then you click "Upload a File" and you select the skin you have downloaded. Then, you click "Upload" and voila! You have a skin!
Did you type your username correctly? It's case sensitive I belive. Also your password, did you type it correctly? that IS case sensitive
Try logging in with the email address you signed up with in place of a username (you may need to request a parent's help for this if they signed you up). You must use your Mojang account to log in, so check that you are using the correct details Hope this helps! Gloss