Alphanumeric characters and spaces only, case-sensitive, maximum 32 characters. Choose a good username, because it will follow you around like a shadow until the sun goes dark and all life, all history, all that ever mattered is gone. If your username is awful - IE it contains hate speech, lovingly detailed descriptions of bodily functions, or other things that make people roll their eyes and say "Oh come on, Internet" - your account will be flushed away.
Type the same password again, so you can be sure you entered it correctly the first time. Although you really, really should have entered it correctly the first time. Passwords are secrets. Never type these words into any other website. Never write them down. Never tell a soul. Promise me.
You know that you're human, and I know that you're human, but just in case you're not human and instead some kind of robot sending spam, how many legs does a tiger have?