I'm answering this on the assumption it will be used for creative purposes--not an actual suicide note.
Theoretically speaking, there a few routes one may go when writing a suicide note. The two basic options include, calling everyone out who ever wronged you, and blaming them for the suicide; or apologizing to your loved ones for the suicide, letting them know it wasn't their fault. However, a suicide note may be used as a codicil, altering a pre-existing will; or, if no will previously existed, it can be the will itself. A suicide note can also get really creative, acting as map or set of clues that sends loved ones on a scavenger hunt or quest, ultimately bringing them closer together. You could go totally left-field and leave a note that is completely unrelated to the suicide--a poem, shopping list, to-do list, etc.