In order to host a server in this game, you have to open the TachyonServer.exe or TachyonServer.jar file inside the Server folder. When it opens, there should be an empty window that opens up. This means that the server is currently up.
Once its on, you just play the game, connecting to the computer running the server. If you are connecting to your own computer, use the address 127.0.0.1 . Otherwise, if you are trying to connect to another computer or server, I would suggest using hamachi to host over the internet.