Ideas for topics come from a myriad of sources - some will be generated by interest in a particular area of previous work, others by discussing issues with peers and academics and some by reading the literature. Initial ideas can also be in various stages of development - some will be vague, others clear and well defined and many will be in the middle of the two extremes.
When thinking about a suitable topic, it is important to consider the implications of your choice:
Can information be gathered locally?
Are you in a position to travel to use various sources?
What are your interests and will this interest be maintained for the duration of the research?
Who will be interested in this research ?
Is it sufficiently interesting to keep you, the author, working for the next 2 – 3 – 4 years?
Is the scope wide enough to be able to ascertain a particular niche?
Is the scope so broad that it will lose direction?
Does it involve technology that is readily available?
Is training in technology and/or software readily available?
In the early stages it is probably wise to give all topics under consideration a title and write them down. Include a brief description of the content of each idea and a plan of how each topic could be developed. Writing sometimes helps define ideas and helps you to plot a course of action. Although the topic may change or aspects of it may change, it is still useful to record thoughts. It is also interesting to look back once the research has been completed and see how the topic evolved!
While thinking about research topics:
discuss ideas with colleagues
browse the literature, especially journals
discuss ideas with your professor -- he/she is an expert within the discipline and can help you decide on an appropriate topic