Get an idea of what you want to study. Ideas might come from hobbies or problems you see that need solutions.
Many ideas come from web sites and books.
Limit your topic and focus your time and effort. As you narrow your ideas, remember to choose a topic that not only interests you, but can be done in amount of time you have. Research Your Topic. Read everything you can find on your topic. Talk to professionals in the field, write to companies for information, and obtain or construct needed equipment.