Well I finally got my hands on ME3 and wanted to try the multiplayer part quick.
But...
I cannot create an account, keep getting invalid username, which is confusing.

In fact I can't even figure out that screen. It asks you to enter your email and password. But there's no double entry for a password which is weird if it was the account creation screen, but I'm not sure if it's even the account creation screen, but it says it's there but where's the login screen? There's only one screen...The whole thing is a contradictory mess and I cannot figure it out. Even the error message helps me zero, because it never asked for the username and only email, but the email is correct.

Can someone please explain to me the process?

Apparently I had created an origin account when origin got released for some reason. Had to reset it's password before the account got active again (The previous password became invalid, you now have to use upper, lower case letters and numbers. All these weird hurdles to get online...

And I couldn't find anyone online to play with which is kinda weird. Online can't be dead already?

wouldn't surprise me wii u ME3 community is pretty small now add in that the multiplayer on ME3 just isn't fun (my opinion) and i think you'll have a hard time finding a full match.