I'm the wet blanket of despair... It's very good that you're asking. It's kinda bad that you're trying. There are many ways to mess this stuff up and doing it for a real live social-security/bank-account site the first time out of the gate is what I would call a really bad idea. Here is a partial list of concerns and ideas-

• All requests with sensitive information (including sign-in) must be under HTTPS configured and set up on a guaranteed host; cookies must never contain customer information.
  • Cookies should be secure and httponly.
• Taint all input. Escape all output.
• Encrypt/salt passwords so they can't be discovered, reversed, or found in a lookup table.
• Encrypt the stored financial and personal data. Keep the keys in a separate location; i.e., not the database.
• Limit all access to all resources to the smallest possible permissions/connectivity needed to run.
• Run everything as an untrusted user so you can't make dangerous mistakes.
• Bone up on PCI compliance. Follow it.
• Read OWASP. Take it all seriously.
• Raise the bid on your contract, or eat the cost, to hire someone who has done all this several times to audit the project when you're close to a production release.

Good luck and stay scared. It makes more secure apps.

Update: changed PCI link to the one grep provided; it's better. Update:update: removed a redundant/awkward sentence.

Update: added OWASP and HttpOnly notes.

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