Well, the game is quite simple...Collect the salvage-able pieces from ancient wrecks, or ships that you've destroyed. You're refuse count (top-left hand corner) shows how much the wreckage is worth and you can use this to purchase ship upgrades, as well as a few other things. While it may seem simple, there is an actual storyline to the game, one which involves you doing a fair bit more than just collecting wreckage. However, I haven't started working on that part of the game yet, so for now, it's just defeating pirates and collecting salvage.

The HUD has three things being displayed: The amount your collected refuse is worth (top-left hand corner), your health and you're speed (top-right hand corner, the green is health, blue is speed), and you're ship upgrades (bottom-right hand corner).

Controls
Arrow Keys - Move the Isaac.
Ctrl - Fire laser cannons.
Space - Magnetise hull (attracts ship refuse).
X - Upgrade the Isaac's engine (if you have enough refuse).