There are essentially three parts to a cover letter:
Opening Paragraph is used to explain why you are writing. If you are responding to an advertised position, state what the position is and where you heard about it. If you are sending a letter of inquiry to an employer (no position has been advertised), state where you heard about the employer (from a networking contact, newspaper article, etc.) and perhaps, why you are interested in exploring opportunities with them.
Middle Paragraph(s) is your "selling" paragraph. This paragraph is all about making connections - connections between your resume and this employer's specific needs. Refer to your resume, but do not reiterate it. Rather, explain why the experiences you have listed would be valuable to this employer. This paragraph is also an excellent place to talk about some of the unique characteristics of your Mount Mercy education degree - number of hours in the classroom, etc. You may also use these paragraphs to highlight a specific interest in the type of school, an interest and willingness to get involved in extracurricular activities, and so on. Always try to answer, "What makes me unique?"
Closing Paragraph includes a phrase to affirm your interest in the position and/or school. Also indicate your interest in setting up an interview. Finally, always thank the reader for his/her time and consideration. Some general professional pointers