EDIT: Pwnt. Seriously, a site handling money of any kind should not be your first project.

Quote

Give a man a fish and you feed him for a day. Teach a man to fish and you feed him for a lifetime.
Chinese Proverb

http://stackoverflow.com/a/195019/804495
https://www.owasp.org/index.php/Category:OWASP_Guide_Project
https://www.owasp.org/index.php/Data_Validation

If you think I'm a bit rude, sorry but that is the point - to show that you need to rethink what you are doing. I would be happy to properly disclose your security vulnerabilities if you didn't choose to make a site handling money (without a clue of what you're doing) as one of your first projects.

People who try to do this need to be discouraged. We don't want another bitcoinica or instawallet.

Also helpful reading: https://bitcointalk.org/index.php?topic=124441.120