Before you can begin your research you need to narrow your selected topic.
For example, if "marketing'' is you chosen topic, "marketing" as a research topic is too broad. The amount of information on the subject of marketing is enormous therefore the topic must be narrowed.
Instead, think of some aspect of the general subject of marketing that interests you, for example:
Let's say that you are interested in marketing to special groups.
You could narrow the topic of marketing even more by adding a sub-topic.
It is helpful to write out your topic as a sentence or a question. Let's say the question that most interests you is:
How do advertisers market their products to women?
This is your thesis statement! Now you can begin your research.