Depends on what actually you test.. If it is going to be consumer banking site, then its a great challenge in performing security testing. As this is very important and has a power to destroy customers trust and may possibly increase the cost on controlling any threats in the future.
Below are the key factors:
Authentication
Authorization
Confidentiality
Availability
Integrity
Non-repudiation
Resilience
You can use tools like Fiddler, which will actually help in tampering the consumer entered data and see if they pass through. You will have to perform on each and every field in each and every sub-apps of the application/website. It is huge to think this way. Alternatively, you can filter out items/fields which are classified as highly sensitive and/or Mandatory. As I say this, I also mean there are different approaches while using different tools. What I know is a little, there are a lot to know about Web Application Security factors.