It depends on how long you want to store the thinned (reduced) paint, and what you used as a reducer. A few days to a week, at most, is probably okay. However, some paints are less tolerant, and may go off in a day, especially if you use a marginally compatible reducer.
Since you just got the airbrush, you can use the extra paint for practice on a paint hulk or scrap plastic. Some recommend an old milk jug or similar container as a practice target. If you think you like the airbrush now, just wait until you've practiced enough to be really skillful.