Creating MMO in Uinity

Guys, im trying to make simple mmo game (based on tank game from tutorial). I've alredy made whole login system (using PHP and mysql). I'm making it to learn as much as i can about creating mmo games, and now there's a question: What's next? I want this game to be something like Moba games - you can login, join lobby and matchmaking finds you opponents. You are able to play against them in real-time mode. To do that, as i relised, i need a server. And here's a main question: How to make simple server which i will be able to use in this project? Is it very hard? and even if it is, where should i start? Any good tutorials, etc?