How to parse syslog input, output only certain text to output file

[havenotaclue]

I have Fluentd receiving syslog from a firewall and writing it to text file. Each line contains lots of comma separated info. How can I parse each syslog inputted line and only write out certain fields to file?
I had a look at docs and can't quite work out how to do this, so I appreciate any example configs. I'm a total beginner...

Thanks for any help.