|Honestly? If you're going to be storing people's financial information (credit card details etc) and you have to ask, then... don't. Or find someone with experience to work with on this project - do not try to wing it yourself. This is other people's money and livelihoods you're playing with. But do you really need to store sensitive data yourself?
You will need to use https / SSL.
You will need to encrypt data securely.
You will need to work with someone who knows what they are doing.
You will need a good lawyer.