Advise - Checking the security of a site

We currently have a website that we need to check to see if it is secure and are after advice on where we need to do so. We need to protect against someone with root access at the host fiddling with the code, and how do we do this? How do we make sure the encrypted data in the MySQL database is secure? We need to know how many concurrent users the system can support as the members log in to do their comparison? We also need to understand what security mechanisms are in the website, its design and deployment so as to protect member data from external intrusion.