The idea of this letter is you just had an interview to get a job and you want to thank the person or you want to thank someone for setting an appointment to meet with you last week. The reason to do this is that it makes you marketable when looking for a job and hopefully that helps you succeed!
Paragraph #1 - Should tell them how thankful you are for them spending time with you and how much you enjoyed your time talking with them.

Paragraph #2 - Should remind them why they should either hire you or continue their discussions with you.

Paragraph #3 - Should tell them the best way to reach you (email, cell phone etc.) and that you look forward to hearing from them soon.

All paragraphs should be at least 4 sentences.