With most services, you mainly create a username, or it is assigned to you. If you have created your username and you forgot the name you choose, you can try your email address instead of your username to access your account. If you are trying to log into a service by using an email address, you can try a number that you have entered at the time of creating your account.
If the above suggestions do not work, most of the services have a feature to forgot passwords and a username that can send your username via your email. Most of the time, a link of forgot username or password is found around or under the username and password fields. It needs to remember that when you are creating the username, it may be case-sensitive when entered. Therefore, you can try to enter your username after turning off the Caps Lock key (if it is enabled).