I agree with Aaron re the airbrush, Jim.
A piece of card, and later, some scrap plastic, such as a cup or food packaging, will help.These have a similar, non-porous surface to model parts, and also allow practice on curved surfaces.
With the card or board, use those cheap water based paints for kids, that come in large plastic bottles, and dilute around 50/50 with water, or until a smooth flow of paint is achieved. These paints, being so cheap for relatively large bottles, are ideal for practice, as you can do a lot of practicing without much expense, and they're also very easy to clean up afterwards.
Start by spraying solid blocks of colour, to any convenient size, building up the density in layers, and starting with a dark colour - black, blue or red being good choices.
Then repeat the exercise using pale colours, such as yellow, to get the hang of really having to build-up density with 'difficult' colours.
This will also give you a 'feel' for the airbrush, and the sensitivity of the trigger, and allows you to practice spraying over an area in even passes, without bending the wrist from the line of travel - this will cause 'feathering', uneven lay-down of the paint, and possibly runs.
Once you're happy with that, then move on to spraying straight lines, circles, waves and so on, and signing your name, just as you would with a pen.
When you're happy with this, then start reducing the 'thickness' of the line, by control of the trigger, perhaps by pressure adjustment, and also by adjusting the spraying distance.
You should eventually get to a point, in not much time, where you can spray a relatively straight line down to the minimum the airbrush can handle, which could be less than 1mm thick.
Repeat these exercises with the paint you will use for models, again on the card, but remembering that results may not be exactly as they should be, due to any absorbency of the type of card/board/paper. Once comfortable, then move on to spraying on the plastic containers, to get accustomed to the different surface, compound curves and so on.
Then it'll be time to tackle the first model which, I would suggest, should be something of either one single colour overall, or a simple scheme with large areas of one colour, rather than a complex camouflage or combination of lines and colours.