Any Minecraft account created after Fall 2012, or one that has been migrated from the old account format, is located on a Mojang account, and logs in with its email address. A very old Minecraft account may still log in with its username.
If you have a Mojang account and have forgotten the email that you log in with, try resetting your password. This will send an email to the address that you have registered, letting you know which address you used. If this doesn't work, try other email addresses that you own. If you remember your username, but not your email, visit minecraft.net/retrievename and try different email addresses that you own.
If you still log in with your username, and have forgotten which name to use, you can visit minecraft.net/retrievename to request an email with a list of any usernames registered to that email.
If you can't get system emails, visit this article or contact customer support.