How can I remove stains? Will whitening strips work? What can I do about this cavity with barely any tooth left? (Photo)

I'm very insecure about my smile and not sure what I can do to fix them. What is the cause of this and what can I do to have a regular smile with no stains? Please help. Also I have a cavity but it got so bad to the fact the tooth chipped almost all the way off around the gym and now it's just sitting there. There's only a little bit of tooth left around. In not sure that there is enough to grab for a dentist to pull. What do I do now?