Destroy enemies and collect as many cubes that appear on screen as you can to earn Arcbucks. These can then be used to buy upgrades for your spacecraft in the hanger. Options include homing missiles, lasers, burst missiles, smart bombs and chain lightning, among others.
A good way to add more Arcbucks to your pocket is to do 'Wormholes'. These are bonus missions which challenge you to complete a specific task, these include gathering all cubes, shooting targets without missing a single one, or navigating through a tricky environment.