I dont know how to play where to get started?
If you've never played Minecraft before, I suppose you can give it a try at their homepage - register a free account, and play the demo they have there for a while:
https://www.minecraft.net.
Anyway, it's like playing with Lego: there isn't necessarily a clearly defined objective in the game (though that might change depending on the server you're in); you just explore, build new things and, when the night comes, you die - okay, maybe it's not really like Lego.