If your teeth are yellowish or not just white enough you can use natural (not barcue charcoal because it can have chemicals and wont be good to your teeth's health) charcoal (I dont know if this is how you spell it sorry my english is not the best)
Just take it, and "brush" your teeth with it
Yes they will turn black...
Leave like that for like that for a few minutes and wash it
Then brush with toothpaste and all...
Edit: Here is a very detailed guide guide, hope you trust me now