I think it depends on the scope of your work. You can set yourself up with some free resources and software, such as Pixlr's online editor software which emulates Photoshop .Most professional-level work is created with professional-level software, though. I would personally start with some Youtube tutorials and if you maintain your interest start shopping around for books. I think a good textbook would be worth the investment, especially if it acts as something that you would utilize if you were in school, which would be your best bet to learn the most (but also the most expensive option).