You have a dual action airbrush whereby you can control both the amount of paint and the amount of air, correct?
REGULATE -- Connect your airbrush to a REGULATED source of air pressure. This is first and foremost in controlling spray pattern diameter. If you do not have a regulator, you can buy one at Home Deopt or Lowes. If you don't want to get one, then sell your airbrush & compressor because you will NEVER be able to paint like a pro w/o a regulator.
VISCOSITY -- or paint thinkness is king. I build alot of scale models and typically I paint between 8 to 12 PSI -- that is all. My paint is normally in the viscosity range of whole milk. Load up your paint cup with whole milk and try it out. I know it soulds wierd, but try it. Get used to what it looks like (how thick it looks) when you swirl the cup around a little.
Most of the time I have to add thinner to my paint. Nothing comes "ready to Run" in paints except some of the Alclad metallic lacquers. If you are using oil based enamels, use artist-grade mineral spirits. If you use the newer acrylics, thin with alcohol or Windex (with ammonia). I do not like the acrylics though.
If you mess up and make it too thin try lowering the air pressure or addin a little more paint.
NEVER ADD THINNED PAINT BACK INTO THE BOTTLE OF VIRGIN PAINT. YOU WILL CONTAMINATE THE BOTTLE AND IT WILL GO BAD IN A SHORT PERIOD OF TIME.
KEEP IT CLEAN -- A clean airbrush is a happy airbrush . . . (cliche, I know, but it's true!) Never forget to clean the airbrush after each color with the appropriate thinner until it shoots clear liquid.
I strip the airbrush down to it's basic parts and clean it out with Q-Tips, paper-towels, and string in the orifices (like flossing your teeth) about every 15 to 30 minutes of painting time. That might not sound like much time, but if you look at how long air is actually being pumped through the brush during any one modeling session it really amounts to nothing.
VENTILATE -- I also have a 240 CFM vent fan mounted in the countertop in the hobby room that vents to the outside. Isolated motor, 12V DC. Got it at Grainger.com for about $110.00 -- do SOMETHING to ventialte wherever youy paint.
If your airbrush works great for a while, then starts sputtering paint in an on-off-on-off-on-off-on-off-on-off like a blinking traffic light, it is DIRTY and time to strip it down and clean it. Sometimes I soak mine in Lacquer Thinner. It works great. Becareful not to lose the small parts and I mean they are SMALL. The hollow tip to my Iwata is $26; one end is threaded and is about 3mm wide and 3 or 4 mm long. Drop that in the carpet and you'll be sorry.
I have an Iwata dual action airbrush connected to three water/oil/particualte separators with a Bellofram regulator. My regulator is a precision unit where I can control the pressure in 1/4 PSI increments. Totally unnecessary, but I had it so I used it!! The compressor is a 45 gallon dual stage 240V unit in the garage. Very quiet and the airbrush runs forever off of one tank. (I use it for air tools in the garage as well.)
I can get a 0.2 mm line with my airbrush if I really tune the paint viscosity and the PSI to the correct settings. But it's rare that I need something like that anyway.
You dont have to have all of the sweet hookups, but a regulator (not a valve) of some sort is a must.
Watch your paint viscosity, and keep it CLEAN!!!
Now practice, practice, practice, and ask alot more questions.