Oh dear, you forgot your password! Don't worry, these things happen and it's easy to recover it.
Firstly log out - go to Account and then click the menu button at the top to reveal the log out menu. Then on the sign up screen you'll see a Login button at the top. Click this to reveal a 'Forgot your password' button under the form.
Please make sure you are using the correct email address you registered with, otherwise you won't be able to recover your password. Also, check your spam folder.