Your style must depend on the kind of writing you are being asked to do, so, as with thesis, make sure you understand what kind of writing this is as you begin your project.
*Adopted and modified, with the author's permission, by James Berg, with help from FYS instructors, from Gordon Harvey's "Elements of the Academic Essay." Harvey's elements emphasize argumentation, though most of them apply to essays that purport to be purely descriptive or analytical as well, and they are modified here so as to realize that potential.
Make sure you know what kind of essay you are expected to write, and how much leeway you have, before you begin work. Your agenda thus won't emphasize your own interest in a topic--your own idiosyncratic motivation or desire, which could just be completing an assignment.
Your articulation of agenda is what you say to show that your essay accomplishes something worthwhile for others interested in your topic.
In the case of an essay in which you are not advancing a particular thesis, your agenda may simply involve providing your readers with a clear sense of what you are doing--a sense of direction--from the beginning of the essay to the end.
Perhaps what you are explaining is especially difficult to understand, and your work is designed to make it accessible. Analysis is what you do with data when you go beyond observing or summarizing it: you show how its parts contribute to a whole or how causes contribute to an effect; you draw out the significance or implication not apparent to a superficial view.
whether a certain piece of evidence validly counts as evidence in a particular case) they should be brought out and acknowledged.
Note that insofar as assumptions are are broadly held cultural beliefs, writers and readers may often fail to notice that they are making them, so writing well often requires attention to others' assumptions, as well as one's own.
In the case of an argumentative essay (an essay with a thesis), agenda often involves the word "but" or equivalent, since, in articulating an agenda for such an essay, you will probably be showing why your argument isn't obvious but requires demonstration or elaboration; how it is useful insofar as it reveals something different from what others might know, expect, or say; how it speaks to a puzzle or conflict that others might experience; or how it has a larger implication that others might not immediately see.
Especially in the case of an argumentative essay, these "others"—imagined or actual—shouldn't be dummies; you need to make clear that their misapprehension or rival claim can be argued for.