Heh, I was being kinda vague to promote thinking about the problem and thus make it easier the next time.
But ok, made you a cap - it's not really commented (for now?), but at the same time it's simple enough: Grab it!
So what you have there is a little tank that has two weapons bays - currently there are two types of weapons - a rapid fire gun and a slower spread shot. Each bay can have either weapon and you can switch between the bays by Right clicking.
To test the pickup system random weapon pickups gets spawned every 3 seconds. I haven't put much thought into them so sometimes you pick one up and the next one spawns right on top of you, etc. But anyways - you can pick up the weapon and it'll replace the weapon in your current bay and become the active weapon.
Have a look around, let me know if anything is unclear. I may improve it in the background, but the general idea will stay the same.