I have multiple emails, and am almost completely sure which email my minecraft account is set up with. I got minecraft a long time ago and do not have a mojang account. It won't let me create one. (My username is KDHandshake) and I put in the password that I use to log into minecraft. I want to change my skin, but can't because I cannot log into mojang/create an account. I'm just really confused.
Your account is unmigrated. This means that it is an old type of account. Unmigrated accounts' skins (and names) cannot be changed. It should be migrated to a new, Mojang account. For this, you will need to know the e-mail you registered your account with, the username, and the password. You can migrate it here. After migration, you will be able to change the skin, name or any other detail of the account.
If this doesn't work, your account has been compromize (hacked). You will need to contact Mojang with your Transaction ID, and they can recover the account for you.
The Transaction ID is an unique ID which you can use to proove that you are indeed the owner of the account. If your Transaction ID is lost, you are unable to recover your account, unless you recover your Transaction ID first. There are ways to do that, which can be found here. The Transaction ID can be found in the original e-mail, that you got when you have purchased Minecraft. It looks similar to this:
Note: This is not an actual Transaction ID. Transaction IDs should not be given out as they can be used to steal your account. Also, 'braintree' can be a different word too.
In case your account was purchased using a Gift Code, your Gift Code is the Transaction ID.
You can contact Mojang to help you with recovering your account here.
You're not supposed to use your username. Instead, try using your email address that you signed up with.
If you're not sure about the email, (like mentioned above) you can try using your transaction info from when you first bought the account to try and recover the email.
It helps to know your recovery questions.