If you are new to this whole web hosting thing or need some very basic questions answered. All usernames and passwords should be between 6-8 characters long. You will be grateful you chose a short password when you have to type it 20 times without seeing it. Conversely, a one-letter password is pretty easy to crack.
Passwords are case-sensitive.
Usernames may or may not be depending upon the application. This is a bad idea in both cases, but even more fatal for passwords. Punctuation is particularly bad when you are initializing software and do not know whether some of its component tools do or do not accept punctuation. For example, UNIX accepts some punctuation but mySQL does not. The troubleshooting can be painful.