Qualifications
These days, you'll need a bachelor's degree to do pretty much any professional work. The same thing goes for this field. A bachelor's degree is definitely required, preferably in something having to do with engineering. If you're the type that aims high, you're gonna want to go for that master's degree, too. A Professional Engineer's (PE) license is required if you're offering engineering services to the general public—and, if you want to make money, you're probably going to have to get one at some point.
An internship is definitely a good idea as well. Most companies like to see that their potential employees have prior professional experience. For solar energy engineers, there is an association called ACORE (American Council On Renewable Energy) which offers a killer internship program (source). Try checking that out before you send in your job application.