The aim of the game is to collect scrap - there are some wrecks around your ship. Basically - jump on a wreck from your ship, steal precious fuel, throw scrap at your ship to collect it, repeat until you win. (or get stranded in space without fuel)
Some other tips here:
1. You can maneouver for free in your collection box, won't spend any fuel.
2. Conserve your fuel, always! Use only when you really need it. You can always use the secondary fire on your manipulator to move around if there are objects close.
3. There is plenty of fuel in the wrecks, grab it with LMB to refill your tank. You can also return to your ship for a refill.
4. Pieces of scrap normally give you 10 points, but there are some special ones for 20 (mostly doors, huge pieces of metal)
5. Best way to get the scrap back to your ship is to grab it with LMB and get just a little bit of velocity in the direction of your ship. Then let go and the first law of motion will take care of the rest. Stabilize afterwards and do this with the rest of scrap.
6. It might be a little hard to notice the wrecks from your ship, try looking for the green outlines of fuel cans.
7. If you're lost as to where your home ship is, take a look at the radar on top of your screen.
8. Keep in mind the third law of motion - if you're moving with a piece of scrap in hand, you can always launch it with your RMB to get a boost in the opposite direction.