Testing Web Application

Hello,

I have just edited a web application for a client, it is based on Access Database and would be using the HTTPS for secure transmission for the client/server transmission of data.

I have carried out the  xhtml validation, css validation and followed the w3 consortium regarding image, alt tags, etc

I don't know of any tester, but I was just curious if any one knows step I should take to check the security of the application and more.

Many thanks in advance