Try to get as smart as possible by shooting neurons. When you destroy a blue enemy neuron, it will emit a small white data packet which will work its way towards you. When you absorb enough of these, you can buy updates (press E to open the update screen). Shoot the yellow neurons: when destroyed, they will emit their information, which you will assimilate. You will be rewarded with points you can spend on upgrades, and you will unlock bonuses.
Use the WASD keys to move, the left mouse button to shoot, the space bar to bomb and Shift to activate or deactivate.