Username needs to be at least 3 characters, and may contain only English alphabet, numbers and underscores. Password needs to be at least 8 characters.
Your email address will not be shown to other users. Make sure you enter a valid address. You'll need it if you ever forget your password. For a profile picture you'll need an email address associated with Gravatar. Email address can be changed later.
Alternatively, you may sign in using your Twitter account. A Twitter account can also be connected afterwards.