When you download the game, there should be 2 folders, one for the client and the other for the server. You must configure and run the server version of the game (I made a tutorial video on how to do all of that) first. If you want to connect to your own server, then open up the client application/version of the game, and in the input field where you put in the server's IP, just type "localhost". If you want other people on your same network/WIFI to connect, you must find out your private IP address (if you are on windows, you can do so by typing in "ipconfig" into command prompt and looking for the IPv4 address column, and tell them to join to that set of numbers). If they are not on your local network and instead their own, you must port forward, and have them join your public address (you can find it by looking up "what's my ip address" on google).