How can i make my parents listen to me to get braces ?

I am 16 am my teeth is kinda dissent , I have a lot of cavities but it doesn't show. My bottom teeth are straight and my teeth is clean not yellow. My top teeth is okay except i have a teeth on top my teeth on my gums on the right and left side. I told my mom and dad but my dad said he has no money and he has too pay for bills and my moms not working right now. The type of insurance's i have is fedlis care. I have junior prom next year and the next year after that i have prom and graduation. My friend who have braces says it takes up to 2 - 4 years max. I need help what should i do ?????????????????