Username is a unique name for admin to identify you. It can be anything or whatever you can remember.

Your display name is the important name everybody will see on your profile.

So,

Don't use your real name as a display name.

Use a fancy name like a nice nickname but one your mum and dad don't know about.

You can change your display name later through your profile at a cost.

Date of birth:

Pick your date of birth, it Can not be changed once your registration is completed.