The three classes available during the alpha this weekend were Hunter, Warlock, and Titan. The races include Exo (robot), Awoken (space zombie), and Human. You can level up quite quickly, but don't forget to explore your entire map. If you do forget, you'll miss out on the many opportunities to gather resources, find loot caches and chests, and revive ghosts. Ghosts are the small, flying robot cubes everyone uses. You'll get things from them like coordinates, missions, grimoire cards, etc. You get experience points for all of the above, including your completed missions (as well as glimmer, the currency of the Destiny game universe). But yeah, shoot anything that moves, like Jittery suggested. It's a dangerous place!