Although there are instructions in-game, here is a simplified version:
Click to move;
Things that looks like circles can make you build things;
Manage your resources: when you don't realize, your crystal might be destroyed!;
Do not let monsters in your crystal. Or else...;
Talk with the merchant, he may have things of your intrest.
TERMINOLOGY:
Crystal - The yellow rotating sprite.
Circles/Slots - Where you can build things
Gen. - Generators