Hi Francois,
Been using a Badger 150 since the 70's so got a little experience with them.
Hopefully you got an owners manual with it.
It not only functions as a double action air brush, when you push down on the trigger you are relasing only the air, as you pull back in the trigger you now control the paint flow. The farther you pull back, the heavier your paint flow will be. Thus you get more control over the airbrushing. When you release, trigger, the spring forward movement shuts off the paint flow,and air. If you mover the trigger forward as you keep the trigger depressed, you decrease the paint flow. Don't worry, with practice it all becomes a natural movement.
It also acts as a single action air brush. Meaning if you set the needle in a set position, it will spray the same pattern each time, as soon as you depress the trigger.
To set your needle is really simple.
1. loosen the back screw and make sure you needle is gently seated in the tip. GENTLY, unless you enjoy replacing tips. Then tighten the needle in place, using the rear thingy. Once this is set, you don't need to adjust it again until you take apart the brush for cleaning.
2. to set for single action, on top, just in front of the trigger there is a tiny little screw. turn this in until you get a spray pattern when you depress your trigger. Adjusting to spray pattern you want. You can now use it as a single action brush, each time getting the same basic spray pattern when you depress the trigger. Of course if you suddenly need to, or wish to revert to double action, in the middle of a spray secession, simply pull back a little on the trigger and you go to double action.
I don't know what set up you got, hopefully the one with the three heads and needles. Don't expect to get tight small spraying from the large (HD) head, and don't expect to get large coverage from the Fine (F) head. For general all round modelling, inbetween, the Medium (IL) head works fine for most general modelling work.
Remember to clean your airbrush and keep it clean. When you remove the needle, be careful you don't bend the needle or damage or bend the point. (the fine one is easily bent) Same thing when reinstalling it.
When you remove the head, there is a little white teflon gasket, becareful you don't lose it, and never over tigthen and smash the gasket when reinstalling the head. Also be careful when cleaning the head, the tips are fragle, and will split or distort easily. (usually the major cause for bad spray patterns).
I use many different airbrushes.
Badger 150, Badger 100G, Iwata HP-B and Paashe VSR90, and the Badger 150 is a great all round modeller airbrush, easy to clean, cheap and easy to get parts for. My original Badger 150 was replaced earlier this year with a new one, and the old one was passed on to a modeller friend. The only replacement part ever needed other than tips, and needles, was a new air valve, and that was 20 years ago.
Good luck, and looking forward to seeing your work here at Armorama.