Registration Page and Subdomains

Hello!

First of all, I’m having issues figuring out exactly how to create a custom registration page in general. Ideally we’d like to keep it relatively short and just allow people to update their profile as they please later on. Fields we’d like to include are just:

Gamer Tag
Email
Password

But I can’t quite get it set up…

More ideally, because we are using subdomains, I was wondering if anyone has a way to let people register to our main page and be able to use that one profile across the subdomains we set up. The end goal is each subdomain is uniquely tailored per game, and since many players play several games, we’d like them to be able to create Tournaments for several games without the hassle of registering for every single subdomain. Ideas? Thank you!