Need help with my tooth.?

i was flossing my teeth and a part of my tooth fell off of the back of my three teeth in the back. please help because i dont have insurance and i think its a cavity because it turned brown a little. and the chips of tooth that fell were brown in color. ive never had a cavity. so i want to know how much it cost to get it fixed. would i need a root canal? or are my teeth going to just need a filling please help. and how much does it cost to get a filling on my teeth?