The book is filled with great ideas for writing essays and for writing in general. Hopefully it is still in print. An essay is the written expression of its author s opinion. Think before you write.
Opinion always comes first.
The thesis of your essay is your opinion boiled down to one arguable statement. Look for relationships. Points that can be made against your thesis. Points in favor of your thesis. The function of the introductory paragraph is simply to introduce the subject and come to the point.