Engage in fierce 1v1 (or 2v2) fights in GeoArena, and customize your ship so that it can kill the best way possible.
Move with WASD, shoot with the left mouse button, and change weapons with shift. Use your ship's special skill with space.
There are also neutral enemies (red ships) that you can farm to level up.
There's a rhythm to battles. They don't necessarily play out quickly. Typically, players stay back and try to level up some in order to gain power before they engage each other.
You could do this yourself, or - if you're cool - you can try to play aggressively early on to catch your opponent off guard.