You can check if you have combat enabled by going to:
Options
Game
Difficulty

(Most of the time, this will be open when you click on options)

Make sure the dot at No Combat is unclicked, so it needs to be black!

Now, launch a map of choice.
When you've spawned, if you enabled the UI, you see in the left corner, >NAV and TGT. Click on TGT.

As you can see, new mysterious red arrows appear! You can go and seek the closest one. Try and get close to it and proceed to the next step

Now that we have approached this drone, we have to get in in our range. In the VR cockpit you will see a giant circle. If you manage to get the drone in there, and it's close enough to you, you're ready to fire and you can press Right Shift in all it's glory. A missile will fire and if you are a master you will blow up the drone.

Some of you actually have friend and play combat with your friend in the multiplayer mode. When a missile is coming towards you, press Right Control to set off flares. This will create a bunch of particles around your craft that will confuse incoming missiles. Which makes them very likely to miss!

That's it, you're now an unresponsible astronaut in a Lunar lander which can fire rockets. Have fun!