If you are wanting to host a server it is actually very simple.
Step 1- Download any map.
Step 2- Enter your name.
Step 3- Press "Host" and it will begin to load.
After it is done loading it will ask you what would you like to be. After you choose just wait and players will soon come.
Your server will automatically go into the server page.
When a player is joining it will say on the bottom left side of your screen "Username is connecting".
Also if you would like to host a server make sure your computer can handle running one and that your firewall is off so players can connect to your IP.