Theysee,
I know in the first paragraph, you would describe tthe position you are going after. You would put in something that says that you think it would be a possible fit for you.
The second paragraph would contain your background that is tailored to the position you are seeking.
The final paragraph would be a brief conclusion, followed by your contact information and availability.
Closing with a thank you for the consideration of candidacy for the position.
Signature.
As you can tell, I went through a lot of this!
Good luck,