The first paragraph is your introduction to the employer and should do a number of things in a short amount of space (which is why the first paragraph can be hard to write). You need to:

• Establish your reason for writing
• Establish your credentials
• Make reference to an announced position (or inquire about possible positions in a particular field)

More Tips:

• If you have talked with the employer before, reference that contact. For example: "Thank you for speaking with me on the phone last week about your position opening."
• If possible, your letter should include a personal reference to someone known by the employer you are writing to. For example: "George Franklin suggested I contact you about the job opening in Sales and Marketing."
• Before writing a sentence like this, though, you need to make sure from George Franklin that your employer knows who he is. Otherwise what is intended to be a referral might work against you. If you are unsure, you might write, "Another employee of Microsoft, George Franklin in Programming, suggested I contact you about the job opening in Sales and Marketing."
• You do not need to include your relationship to the personal reference whose name you are using. S/he might be a family member, a professor, a friend, an alum, or even the sister of your uncle's former college roommate. Whomever you want to mention, however, you must have their permission. Ask that person directly, "May I use your name when I contact [name of hopeful employer]?"

Sample cover letter #1

Sample cover letter #2