I'd recommend playing the tutorial to get an idea of how the game works, and look through your controls and other options so that you can play the game more comfortably and to your liking
The main goal of the game is to build, but depending on the server you go to, or the gamemode you decide to play, you can also race in vehicles, battle against other players in deathmatches, do RP things, etc
You can also browse the 'Blockland Files' section of these forums for various ways to modify your game, although, for quick and easy add-on installation and management, i would recommend downloading
Blockland Glass and put it into your game's Add-Ons folder