Move: WASD or the arrow keys. Your wisp glides with a bit of inertia.
Attack: left click toward the cursor. E switches between the weapons you unlock as you grow.
Shield: hold right click to raise a shield that blocks projectiles (it drains energy).
Dodge: Space for a quick dash (costs a little light).
Grow: absorb the light on the ground. The brighter you are, the higher you climb the leaderboard and the more weapons you unlock.
Portals: walk up and click a portal to change world (forest, lava). Enemies there are much tougher.