I need to see if I am on the right track-

I am to write an essay on why I want to become a nurse. I am trying to get into their nursing program.

Do you have anything written? If not, my strategy on writing such essays is to get a sheet of paper and a pen and just write whatever comes to mind relating to the subject. Not sentences just words. Then those words just turn into sentences in my minds and I write them down.

If you have something written, you should post it here and people can help you better

If i were you I would just work out what you want to write before you actually start writing and get your thoughts down. (kindove like what the first comment said)

Try free writing, just grab a pen and paper and write anything, forget about grammar or punctuation, just scribble. While writing think why you want to be a nurse, think about your past experiences and how they affected your choice. you'll have a bunch of ideas now which will help you think of a bigger story, and theme for your essay. Use the good parts you wrote as a base for the essay and let them grow more detail. now you have your first draft, let it sit for a couple days and come back to it. This is what I always do.

the secret is to write, rewrite, and rewrite again.
you can get a lot of help here if you post your draft (i did), some people will even rewrite or edit the whole thing.

and bimongol's way works too. I also recommend it.

good luck with your writing :)