I'm trying to send out letters to get an advisor, but what have you done that you think i should include?

I've written 3 theses (ugh) so I should be able to help, but what exactly are you asking for advice on? If it's about getting an advisor, make sure each letter is personalised to whomever it's going to - say why you're interested in their work. You should also state how you're going to be an asset, what you're going to add to the body of literature on whatever topic. State your personal interests which should be close to but not exactly theirs. List any experience you already have (research, internship, relevant classes) and how it has changed how you think. If you've taken a class/done work with them make sure to mention it towards the beginning (i.e. what class it was and when you took it). As long as you sound professional and engaging you should have no problems.