If you've got an old pickup truck sitting in the weeds rusting away but you would like to do something with it, and it still has a decent engine, then you should get started fixing it up. You could make the process of restoring it (or at least getting it into good shape) a project you work at on weekends. It's totally doable on a budget, as long as you know how to source good used auto parts, and are adept at searching salvage yards for the right materials.
If you're looking to get started working on your car, but have never done any serious automotive work, then you might not be ready to start with something such as rebuilding a transmission or replacing an alternator. However, changing your car's oil is a great way to start learning about your car and working on it. It's something that needs to be done on a semi-regularly basis, so you should learn how to do it anyways and save yourself the cost of bringing the car in to a shop.