Make a paste out of lemon juice and baking soda or make a paste out of strawberries - brush on teeth.
Weakened enamel. Scrubbing a citric acid on to your teeth with an incredible abrasive material is GOING to wear down the enamel on your teeth.
And if you do this - you're going to notice your teeth get whiter, because you're removing the stained enamel - but the more you do it you'll notice your teeth start to yellow - because the top layer of your enamel is white, but under it your enamel is brownish/yellow.
Just go get yourself a whitening kit. Yes - they cost money. But I found a whitening gel with tray at the pharmacy (I believe it's available at Walmart too) for $15. You just have to be willing to find the deals. The Lustre whitening kit - including an accelerator paste, rinse and light - is $25. Or get strips, or go to the dentist. I'm sure you can find something in your budget that won't destroy your teeth.
Something else to keep in mind: go for regular cleanings, floss, brush at least twice a day etc etc.