The suicide note or death note usually, states the reason(s) for the writer emotional and physical state and reiterates the suicidal tendencies that, if acted upon, will result in an attempt to end the writers life.
As to what actually goes in the note is more of a personal nature. Some only wish to state the reason why they are to destroy their existence from the world and justify their actions. Others may wish to state how they want their body to be disposed of, whether by cremation, coffin burial, or any other form that they may have chosen. Yet others would like to leave their possessions to said individual(s) whom they have addressed by name in the letter.
Beyond these archetypes the possibilities are limitless. The suicide note could contain anything. Perhaps it is the lyrics of a song the writer liked, maybe its a hate note to an individual, it could even be a blank note with the words 'suicide note' written on it for the sole purpose of comedic affect. No matter the reason, a suicide note, if truly meant, is composed by the writer with all and any emotion, thought, idea, or reason that the writer has chosen to leave behind for whatever reason.