That is a very open question, and cannot really be answered as such. There are many classes offered that will help with basics such as this.
I would suggest reading the PIX documentation (and configuration examples:
http://www.cisco.com/univercd/cc/td/doc/product/iaabu/pix/pix_sw/v_63/index.htm
Also, the PIX firewall can do some security, but it is not an Anti-Virus, IDS (beyond basic funtionality).
If you have any further questions, please try to be specific as possible, and provide as many details as possible. Hope this points you in the right direction!