Mechanical writing tools like computer. Never use hand writing for cover letter.
At first, you should address the letter to a specific person and calling by whom is not exciting so that you should know who you are going to send the letter.
Third: demonstrate what you understand about the company activity.
Forth: relate your past related experiences, to the job you want to do.
Fifth: if you respond to a job posting application, check your address to specific address mentioned in the post.
Step six: if you want to attach any data mention that in the letter for not being forgotten by reader.
Step seven: after that thank a reader and finish by sincerely or best.
Check your letter and email it.
You did it!