I hope you can come to enjoy the study of history. It truly is fascinating. And the fact is that no matter what your major is, the writing expectations are the same, to have a clear thesis that you can support with good arguments. Let's talk about Mary, Queen of Scots, and then about how to write a paper like this. Your writing center has not let you down, really. But they cannot help you if you don't even have an idea to work with.
Mary lived through some of the most interesting periods in English and Scottish history. If you do even cursory research on her life and times, you can see this. She became a queen when she was six years old. There are plots and murders; there are imprisonments and arranged marriages. This is a period rich in the drama of history.
Since you have already done some research, you need to ask yourself what you find interesting about Mary. You must ask yourself what more you would like to know about her. You must ask yourself what you find admirable about her or what you find despicable about her. You need to explore your ideas about the times she lived in. She may have been a product of those times or have done something daring and modern in her day. She is a female, trying to get along in a male-dominated world. She is a devout Catholic, trying to get along in a Protestant world. She dies accused of attempting to assassinate Elizabeth. You might very well wonder if she was guilty. You need to decide what strikes your fancy about Mary and her world.
Out of all of this, an idea will emerge, and you will need to support that idea, first in a thesis statement, and then in the paper. Let's take, for example, the idea that Mary might have been innocent of conspiring to kill Elizabeth. If that is your thesis, you will have to support it. Writing a paper for history can be like being a detective. Mary might have had enemies who framed her. The court may have relied on circumstantial evidence or no evidence at all, for that matter. Elizabeth might have gained considerably by getting Mary out of the picture. All of these would be the means of support for such a thesis. The idea is to then state the thesis and the supporting points in one sentence, at the end of your introduction, saying essentially, "Thus and such is true because of A, B, and C." That's all any thesis statement is, really. Then your A, B, and C must each be developed into a section or body paragraph, with your evidence that shows that your thesis, your assertion, is true, or at least feasible. You end with a conclusion, which is a review of your thesis and points.
The study of history may or may not be for you, but a coherent and organized research paper is a normal expectation for most college classes. The sooner you learn how to do this, the smoother your college experience is going to be. There must have been something about history that appealed to you, that caused you to choose this major. Don't lose your interest the first time you hit a little bump in the road!