Hi Raj,
There are a couple ways to do this. If you want to send everything to a remote syslog server you'll need to get a shell on the box and edit /etc/syslog-ng/syslog-ng.conf. You'll need to add this line to the log facility destination in question:
*.* @
If you need to do anything more complex, syslog-ng's configuration guide has details for expanding beyond the basic.
If you want to do it from an iRule for a specific event, you can follow the syntax laid out in this wiki article:
http://devcentral.f5.com/wiki/default.aspx/iRules/log.html
Hope this helps. Happy logging!
-George