Mary Shelley suggests two major themes for understanding the nature of humanity or what it is to be human.

The first is hinted at in the subtitle of the novel, in which in she calls Victor Frankenstein the "Modern Prometheus." Just as Prometheus overstepped his bounds when he gave fire to mankind, so Frankenstein oversteps the boundaries of what it is to be human when he creates life out of inanimate matter. One of Shelley's themes/messages is that the creation of life is God's prerogative. Victor is punished for daring to presume he could play God.

Another strand of exploring what it means to be human is the relationship between the inner and the outer self. The creature might look repulsive and monstrous, but, as we discover, he has a human heart and soul. Like any human, he longs for love, acceptance, and companionship. He needs the nurture of a parent figure. When he is rejected and made outcast, he, like any human, reacts with anger and despair.

Out of these ideas, you ought to be able to easily fashion a thesis statement. One idea would be something along the following lines: Mary Shelley shows that human beings thrive when they don't overstep their human boundaries by trying to play God, and they suffer when they do overstep these bounds, as Frankenstein discovers. Another possible thesis would be the following: "A human being, according to Mary Shelley, is defined by his soul, not his outward form, as Frankenstein's creature illustrates."

I think that one of the primary distinctions about what it means to be human is the idea that creation and destruction are very closely linked.  Victor's power of creation, in terms of being the "maker" of the monster," results in destruction.  Residing in what he so wanted, there was a definite aspect of destruction present.  What he so longed for and sought after resulted in becoming a source of resentment and intense desire for negation.  This seems to be a reflection of the modern conception of tragedy in that the appropriation of the world in accordance to one's subjectivity results in a futile end, at best.  Victor's narrative as well as his relationship with the monster ends up demonstrating how there is a dualism present in what human beings do, and being mindful of this becomes a part of consciousness.