Welcome to PF;
You will need to start by finding a University that has a post-grad program in the areas you are interested in and then study the prospectus they publish.
Typically renewable energy comes under geophysics while solar energy would be in electrical or materials engineering or solid state physics. You can also study engineering and specialize in energy later. A lot will depend on whether you want to build things or study them - it sounds like you are more interested in research (finding new stuff as opposed to improving existing stuff) so I'd say concentrate on science over engineering. So it looks like you should go for physics and maths as an undergrad.
Now yu know where to look - check out the courses in more detail.