Without any specificity to this question, there can be little specificity to the response. (i.e. What genre of literature is being discussed? Which specific work?)
To attempt a suggestion here, when one writes about a literary element in a work, usually it is the most salient one or the one that is most critical to meaning. That is, if one writes about a short story or novel, one considers which element is most essential to the total effect of the narrative. Often a character is intrinsic to the theme of a narrative; he or she contributes to the development of this theme as a dynamic character who undergoes psychological and/or physical and emotional growth and change. Certainly, for a work such as "Hamlet," the Prince of Denmark, Hamlet himself, is absolutely essential to all the other elements of the play.
On the other hand, it is often a fresher, more interesting approach to write about a less obvious element that also contributes subtlely to the meaning to a work. At any rate, the thesis must state why this element gives meaning to the work and why it is effective. (Always consider what you can support with details and specific examples when developing a thesis.) Perhaps, in the introduction a passage can be cited that indicates the importance of this element.
If writing about a poem or a play, there are other elements to consider besides those in narratives, of course. For instance, in the Expressionistic drama of Tennessee Williams, "The Glass Menagerie," the use of lighting is essential to the play as it creates illusionary effects that express several meanings. In poetry, literary devices such as metaphor, rhyme, diction, etc. contribute to the meaning and effect of a poem.
enotes provides assistance in the Essay Lab, and the how-to topics. Three are cited below for your convenience.
This question is a bit vague, but I will do my best to give you some helpful information. First, you need to be able to identify elements of literature, and I have provided a link that should be of use. Second, elements of literature do not exist in a vacuum. They are for a purpose. So, you need a thesis statement that asserts what a particular element of literature does. Third, you cannot arrive at a thesis statement without being able to think about how an element of literature operates in a specific piece of literature or in a few pieces of literature.
Now, to be more concrete in my advice, let us suppose you want to write about the use of setting. You might choose a few poems, stories, or novels, or even one of each and analyze what contribution the use of setting makes to one or all. For example, what does the setting of The Great Gatsby contribute to this novel? Once you are able to do so, you will be able to formulate a thesis statement quite easily.
Remember, though, that a thesis statement is only as good as its support, so you must be able to offer good examples of how a particular element contributes to specific instances in literature.