.......What causes lisps?

Ok so some ppl say i have a lisp. most of my friends don't notice it. most PEOPLE don't notice it. its really.....there? but not really. its just kinda.....barely noticeablee unless ur intently looking for it when i speak. usually. and i want it to stop. :( im not going to go through middle school and high school with this. help? oh and my friend said that it has something to do with my teeth and how close they are to my tongue. im getting braces soon. will it help?