Go out and buy some acrylics, they wash off with soap and water, and can be completely removed, even when dry, with Windex with Ammonia. Then buy the cheapest kit you can get your hands on and build it without trying to detail it. It's going to be your practice target.
Then practice, practice and practice. Then wash all the paint off with the Windex and practice some more. Spray wide area, and narrow area, play with the airbrush and see what it can do. The acrylics are easier to clean up and dry fast, so you don't have to wait to do multiple coats. They also tend not to annoy SWMBO like the smell of enamels do, though your results may vary.
This will give you the confidence you need to do a "real" model.
And don't forget to practice!