Your best bet is to start with Hansteen's manuscript:
This is also the basis of his book,
The Book of PF which contains some information not found in the online manuscript. The second edition of
The Book of PF is to be released in September, but given the number of changes which have been made to
pf(4) since the first edition was released, the first edition is now only marginally recommended.
If you are not familiar with the , this is a
must-read document as it covers all current features to
pf(4).
Being new to OpenBSD, we highly recommend you familiarize yourself with the information found in the offical FAQ:
...as this is the definitive source of information on OpenBSD outside of the manpages.