Whether the topic has been assigned to you or not, the subject you will write on must be suitable enough to be adequately covered in 1500-5000 words or 8-20 double-spaced typewritten pages. Hence, a broad subject must be limited from the outset.
If the topic is within the area of your course, the first place to start research is your course text. Choose a subject that genuinely interests you since you will be spending a lot of time researching it. Having found the subject, ask yourself, "what aspect of it seems most inviting?" The research paper is a study in depth, so the smaller the area to undertake research in, the easier your task. To take an example: your instructor has assigned the general topic "Poverty in Canada" and asked you to write about one aspect of it. Historically and geographically that subject is virtually inexhaustible. Even if you limit it to "Poverty in British Columbia" or "Poverty in Vancouver" it is still impossibly broad and would take a year to write a report on. So, narrowing your focus, you may settle for a manageable topic such the "The insufficiency of welfare payments in providing for the needs of a four-member family in Vancouver" or "A typical low-income family in Vancouver and their immediate problems in maintaining themselves at a subsistence level." As you do your research, you will probably get a clearer idea of how your topic can be narrowed and focused.
If your teacher lets you choose your own topic, avoid topics that have already been exhausted (ex. abortion, smoking, euthanasia, capital punishment). It is too easy to repeat the same old hackneyed sentiments about these topics. So avoid them unless you can think of an absolutely fresh approach.
Allow your research to be prompted by a question. For example, "Should immigration into Canada be determined by one's education and job skills rather than by kinship?" Or "What effect do small welfare payments have on a family in Vancouver?" Your research will then be focused toward seeking out answers to your question and perhaps proposing your own solution to the problem.
Although you may have a tentative thesis before you begin your research, (you may, for example, suppose from the very start that welfare payments are insufficient to provide proper food and clothing for a family of four) your thesis may change and, in fact, you may arrive at the opposite conclusion after you've done all your research. That is why it is necessary to keep an open mind and avoid foregone conclusions. Don't try to bend facts and evidence just to support your own predetermined views. Allow your views to be flexible and open to change, to change as the evidence persuades you.