One good way to motivate a teenager is to make some kind of reward available to them for doing certain things like getting good grades, keeping a job, or helping out around the house. Most teenagers don't want to do anything unless there is something in it for them in return. If you teach them that being ambitious leads to positive things, you help to instill that value for life. It doesn't have to be a cash reward either, it can be use of the car, going to a favorite restaurant, or anything that they like or enjoy. I know a lot of people say you shouldn't have to reward basic stuff like that but it does really work and helps them to develop a good work ethic once they start to see the benefits.