How should I fix my teeth for my discolored and stained teeth after braces?

Personal consult. There are many ways, some simple and inexpensive and others more complicated and expensive. You need to consult with a dentist who can actually examine you and discuss your options. The internet can afford many people with excellent information, but this on one of those situations requiring a one on one consultation.
Bleaching. It really depends on the type of staining. If you have white scarring then you may need a procedure called icon or even dental restorations such as fillings, veneers, or crowns. If the teeth are stained darker then maybe a bleaching will help. You should consult with your dentist and orthodontist to discuss options.
Scar vs stain. Discoloration usually due to marginal oral hygiene while braces were on. If teeth stained, the discoloration can be removed with a good dental cleaning perhaps followed by bleaching. If the teeth have been decalcified (damaged) the white discoloration will need restorative treatment. Ask your orthodontist to coordinate with your family dentist for the best rx plan.
Baking soda. Is a gentle dentifrice that works very well, and you probably already have a box in your fridge! if that doesn't work, consider a professional cleaning and/or whitening.