Firstly you need to make sure that you the correct amount of Drone Skill for the Revision of the drone you wish to use.
When you first enter combat you will need to select which drone you wish to use from the drop down menu on the combat screen. After selecting the Rev of drone you wish to use you need to click the Launch button.
This will launch your selected drone into combat at the cost of 1 Action Point. Now everytime you fire your weapon the drone will also fire at the enermy.