As to thinning, go to the manufacturer's website and see what they recommend for thinning. There is no general rule, since every brand of paint uses different polymers, binders, solvents and pigment ratios. Also, different colors have different pigment weights and grinds so they need different thinning ratios. My favorite paint is almost water thin straight out of the bottle and still requires thinning (note: they are solvent based, not acrylic). Some colors thin at 1:1, but some require 1:2 (paint:solvent) to airbrush well.
And be aware there are many, many thinner formulas for both solvent based and "water based" paints. My favorite acrylic thinner is 60% distilled water, 39.9% denatured alcohol, and 0.1% surfactant. I use an agricultural surfactant, because it is non-foaming; other just use liquid dish soap.
And, as others have said -- practice, practice, practice! I actually enjoy the practice time. I do it before every "real" paint session if I have been painting for a couple of months. I agree with the food color in water as a practice medium. Try practicing with red and blue, yellow is tough to see, green is "graduate level" practice.
Its also a good idea to clean all parts (except wood and plaster) before painting them. With resin castings, its an absolute requirement. With plastics, it can't hurt, and you never know how much mold release is left on the parts when you receive then. I use Dawn, but I'm sure any soap will work. Clean in warm soapy water using a tooth brush to scrub the parts. Rinse them in clean warm water, and wear gloves to remove the parts from the water. I prefer nitrile -- $6.50 per 100 on sale at Harbor Freight. Latex will do, but avoid the "rubber kitchen" gloves. And leave the parts on the sprues, it makes them easier to handle.
As to the use of primers - I generally don't. The only time I will is if I need an intermediate color coat: Black plastic that is going to end up white or yellow. That said, I do use primers, but not as primers. I find red oxide primer a great color for brick, and gray primer for concrete or concrete block. It is treated as just another color in the paint arsenal. I get mine at the local O'Reilly Auto Parts.
As Alan said, stay with model paints for now. The Plaid/Apple/Folk Art paints, while quality paints, are intended for the craft brushing crowd. I can air brush them, but I got my first air brush over 35 years ago. And as I said in another air brushing thread, get me started and I'll talk your ear off.