Use the Arrow Left & Right keys or A & D to move. Pressing the Space Bar will make your character jump. Pressing Left Control will let you flip gravity.
Your goal is to collect all 9 stars hidden around the map. Avoid the lava and use the blue platforms to auto flip gravity.
You can only flip gravity once before landing. However, you can use still use it even if you're not grounded, assuming you havn't done it before. (You can tell based on your character colour!). If you walk off a platform, you can press the flip gravity key at the right time and get on the other side. When you switch gravity, your momentum continues in the direction you were going. If you switch gravity and use this momentum, you can gain more height.