This is a really vague question. Do you have a firewall?, for example. Does your company have an ftp site, customer web services? Are you UNIX only or lots of different OSes?
Most UNIX anti-virus applications do work reasonably well. And there are open source ones like clamAV, that are free. Go to
SourceForge - Download, Develop and Publish Free Open Source Software
Based on your question and seemingly limited understanding of what you need: consider finding a security consultant. They do this for a living. All you need to do is talk to past customers of theirs to verify the quality of the consultant's work.
Or do it all by yourself and hope you get it right. Download and install something like clamAV. You decide.