In Fly or Die, you start out as a fly and must work your way up the food chain by feeding on objects and other players. This is a double-edged sword though, as becoming more powerful can limit your food sources and make you a target for players higher up the food chain. The higher you evolve, the more food you need to reach the next evolution. The game's goal is to reach max level and become the Grim Reaper.
The game screen contains several useful bits of info up top including your current EXP and the amount you need to evolve, your water and oxygen meters which are important for survival, and your special skill. Speaking of which, each creature gets one special skill. Using this skill will help you to hunt for food, survive, and evolve.
Also, keep an eye on your water and oxygen meters. You must go into the water to drink to fill your water meter while your oxygen meter depletes from being underwater or in the upper atmosphere for too long.
Your creature can also fall victim to status effects such as burning, poison, and infection. These effects cause your creature to lose health over time until you purge the effect by going into the water.