Why not turn to Walt Whitman's "My Heart Leaps Up When I Behold" in which the speaker reflects how it will be when he is old and "The Child is father of the Man." According to Dr. Erich Fromm, author of The Art of Loving, the purpose of a parent is to direct the child towards incorporating motherly and fatherly love into himself so that he can be an independent and mature adult. He, then, becomes the parent to the parent as Whitman says.
To be a father is to teach the child what he should be. This is an active role, not the mere role of a sire. With so many delinquent fathers nowadays, you may wish to stress the active part that fathers must take in their children's lives, for to be a father is to give direction and support. In addition, you may wish to consult statistics on how successful children are who have real fathers, men who are truly involved with the psychological, emotional, and spiritual growth of their children.
I think the previous post had some great suggestions. You could also deal with the feeling of being a father as expressed through art or literature. For example, the feeling of parenting as expressed in King Lear, or in films such as "Field of Dreams" or "Quiz Show," and in songs such as "Cat's in the Cradle." These experiences could form a nice thesis statement about how being a father involves pain and a sweet sense of sadness in one's consciousness. Obviously, if you have personal experience, this would be integrated in any thesis statement, or provable idea about the nature of parenthood, but I think that a writing sample of parenthood or being a father has to include some type of reference point that is not simply subjective experience. In broadening your work in this manner, I think it actually reaches more people and allows the points to be better understood.
There are any number of possible themes for you to concentrate an essay about being a father on. You could choose how difficult it is, what new fathers need to know, what you love about being a father, etc. There are just so many angles with that topic, and you should make it personal if you are, in fact, a father.
As for a thesis, it will depend on what theme you choose. One example could look like this:
It is impossible for a person to prepare himself mentally and emotionally for being a father, as it is likely going to be the most challenging, frustrating, wonderful and rewarding experience in a man's life.
Just one possible suggestion.