Minecraft – Will the email address I use for registering on Minecraft.net be the username

If your account is actually a Mojang account, which I believe all new accounts are, you should actually be using the email address you signed up with in the username box.

Counter intuitive I know, but they still haven't changed it from when you actually used your username.

If you are already using your email address to log in

The login servers may be down. Always check help.mojang.com when you have login problems!

Minecraft uses a single instance, worlds aren't seperated by account, any person logging in can use any world on that instance.

You can use a tool like http://multimc.org/ to create different instances of Minecraft avoiding this problem (each person logs in to their own instance and has only their own maps).