A Resignation Letter doesn't need to be long or detailed, you simply need to state the facts. You should typically include the following in your two weeks' notice letter:
Your contact information
Your boss's contact information
Your resignation date (the date when you're planning on handing your employer the notice letter)
Reason for resigning (can be as simple as "I have chosen to pursue other opportunities")
Notice period or last day of work (usually two weeks from the day you deliver the letter)
A closing salutation followed by your name and signature