When you open it up, in the top right corner (At least, there it is on laptop) there is a box with "choose name" on it. Screenshot:

Click on it. Then, type in the username you want to use, and then the Password. If the username you chose is in use, it will try to sign in to their account, so chances are that you will get the password wrong. If the username is not in use, it will register that password to the username, and you will have an account.
I play on my laptop, so if the process is different for mobile, this might not work.
If this does work, then I look forward to battling you.