Your user ID should be something that others can't guess but is easy for you to remember without writing it down. It's a good idea to use something personal that only you would know.
You should also consider:
Your user ID must be 6-16 characters, is not case-sensitive, and can contain any character found on your computer's keyboard except spaces, apostrophes, quotations marks, and back slash
As an added security measure, your user ID and password cannot be the same
Your user ID should use a combination of letters and numbers (e.g., jim14my or my2dog5is), but avoid having numbers at the beginning or the end
Avoid dictionary words, family or pet names, your birthday, your anniversary or your social security number