Use the mouse and click on the Up and Down arrows button in the bottom panel to rotate the space gun or down. Then click on "Click to Fire" button to fire. Each time you fire your gun gets hot, and is shown by a bar increasing, when it gets too hot you cannot fire anymore!

Once you have built a droid, you can select the droid and make it "Attack" or "defend".

Selecting "attack" makes a droid attack the nearest enemy, always moving toward the enemy base.