Learning to use an airbrush is a large topic and there are other sources to help with all the different types. Learning to mix the paint is another big topic and comes with experience. The big issue there is that there are a lot of different types of paint to use and you have to learn what works best for you. When I airbrush, I use the cheap craft acrylic paints you get in craft/hobby stores and they work fine for me.
If the paint won't stick to what you are painting, you will need a primer. You don't always need a primer though some say you do. A primer is only a layer between your subject and the paint that is compatible with both. If you are painting a printed kit, I would assume you could use a primer. I bet you could get away with a simple clear acrylic coat, preferably by Krylon. Light coats at first, but make sure you fully coat the model.
I learned to paint on plastic in the 80s and brushed with a lot of air and with Polly S paints that were lightly thinned. They were not very runny like many tend to use. I got a lot of clogs, but they worked great for me. Some will tell you that you need to thin to a particular percentage, but that is not completely true. It depends on the type of brush, the air source and flow, the paint, and other things. Some paints might need a flow chemical to help. When using acrylics some paints are opaque and some are transparent and some paint companies do not tell you which they are. Delta Ceramcoat does tell and they work well for me.
I suggest you practice a lot before you attempt to paint a paper kit. It is common practice in Poland so it does work. Get some card stock and practice, changing the variables until you get something that works for you. Remember you can use a regular brush for touchup work or to paint markings and letterings. You might need stencils but I rarely used them. Depends on what you are trying to do. You can get a coloring book, scan and print onto cardstock, then airbrush the image. Great practice. Practice and experiment. Try this on models you print yourself to get your system down, but remember a printed kit like GPM has a different finish so you will need some kind of primer to keep the paint from running. Practice, experiment, repeat as necessary, there are no shortcuts in this skill. Using the brush is a skill to learn, the art comes from what you do with it.