This topic will be your constant companion for several months -- it should be fascinating for you and hopefully interesting to others.
How do I decide on a topic?
Talk to professors, other students, and people who work in your field to find ideas. Think about issues, events, or problems in your field of study which have caused you to be curious. Ask a librarian to point out the annual review of literature and major journals or journal indexes in your field to see what kind of literature or research is current. Recent encyclopedias focused on your discipline are also a good place to find ideas.
Is the scope of the topic manageable?
Think about the time available to complete your thesis. Your topic should gradually narrow from the general to the specific by asking and writing down the questions which interest you as you begin reading. For example, you probably do not have time to study the history of photography in 19th century America, but you may have time to research the influence of photgraphy on the interpretation of a particular event or the work of an individual.
What is my specific question?
As you continue reading, think about framing a specific question which will allow you to explore deeply for original ideas. For example, "What influence did the photography of the 19th century geological surveys have on the American conservation movement?"
Will I find the research materials I need?
Begin to answer this question for yourself by doing preliminary searches in MIDCAT, WorldCat, and the major specialized indexes in your field. Also, discuss your thesis question with your thesis advisor, and setup a research consultation with the Reference Librarian (Library Liaison) for your discipline.