2 Answers

What kind of letter writing? Personal or business?

Block is simplest.  Everything is on the left margin

Date
(Space down about two inches)
Name of recipient
Address of receipient
City, State and Zip
   (Double Space)
 RE:  Topic of letter
  (Double Space)
Salutation  (Dear whomever):
   (Double Space)
Body of letter.  Double space between paragraphs. A paragraph has anywhere from three to five sentences, normally.

Double space between pararaphs. Watch you spelling, grammar, and capitalization. Verb tense is important.  Neatness is important.  
   (Double Space)
Closing (Sincerely, Yours truly, Fondly, etc.),   

WRITE YOUR NAME AFTER THE CLOSING.

Your name  (goes 4 lines down from the Closing.