Hi! I'm not a dentist but I'd like to go into it, as well I'm probably basing my EPQ on cosmetic dentistry
Here's some techniques
1) Toothpastes. Yes, some toothpastes have been proven to make your teeth whiter, various Colgate products have tooth whitening abilities, but the difference is not massive immediately. Costs the same as a normal Colgate toothpaste.
2) Tooth whitening strips. You have to get used to these, but they are strips which you put across your teeth and leave in for half an hour, then take out. The difference is visible immediately after use, but it does wear off during the day. As well, your teeth sometimes get white spots on (these don't last long at all though). If you use the strips, the chemicals from the strip mix with your saliva, and first time use you can get a bit of pain (from an article I read, haven't tried it myself). Standard strips don't cost much, obviously some brands will differ with prices.
Those two you can do on your own, but if you want to have them permanently white you'd have to visit a doctor and pay a bit more money to either have veneers, surgery or a tooth whitening retainer, by which you fill with chemicals that whiten your teeth.
Hope this helps
Mo