It sounds corny but the best thing to write is what's in your heart. Depending on your situation (and what your family/ friends know about your situation), your note can encompass a lot of different things. You can give your reasoning if you want to, or not, that's up to you. You can be as vague or as detailed as you wish. You can look up suicide notes that people have left, that may give you some insight. It is a very personal thing so obviously everyone's will look different. I would defienelty mention (a few times) how much you love them and that it is not their fault. Let them know how sorry you are to leave them, and that they don't deserve this sort of pain. You deserve to find peace, and I am so sorry that it's come to this.