Xnews Scoring
Scoring is one of the most powerful features of Xnews. It's possible to use Xnews without knowing much about scoring, but eventually you will want what scoring offers. Hopefully, some of the links here can get you started.
Primary Docs
Your Xnews directory has some documentation about scoring, and that's where to start. In case you'd rather read it on the web, here it is.
The Fine Manual for Xnews, scoring section
Written by Luu Tran, this is the first thing you should read about scoring.
scoring.txt, the Intro to Xnews' score file
Written by Luu Tran, this is the second thing you should read about scoring. It and the manual may be all you need to read, depending on your present needs.
pcre.html, the Xnews Guide to Regular Expressions
Xnews recognizes Perl-compatible regular expressions (PCRE) in the score file. Luu Tran has given you an abridged, slightly annotated copy of the PCRE man page. (The unabridged PCRE man page is also available on the web.) If you are new to PCRE, it's not a very easy read, but the more you learn, the more valuable it becomes as a resource. There is also a section in the manual about PCRE, and it's an easier read.
Examples
The Tech Zero's Usenet Dumping Ground (archive.org copy)
Tech Zero has put his score file online for you. If you look at it and pcre.html at the same time, much will become clearer. As you learn more, you will be able to glean all sorts of ideas from TZ's work. He's also got an Xnews groups.ini file and a Hamster scorefile to take ideas from.
Murray Peterson's score file (archive.org copy)
Along with the other useful stuff linked from his main page, Murray provides a sample score file for dealing with spam in binaries groups.
Software
The Regex Coach - interactive regular expressions
As the website says, The Regex Coach is a graphical application for Linux and Windows which can be used to experiment with (Perl-compatible) regular expressions interactively. You can believe all the blurbs at the top of the page; this is an extremely helpful app.
Regular Expression Tutorials
Regular-Expressions.info
This site has both a quick start guide and a full tutorial. The latter is extremely thorough. (Note: the author of this guide and tutorial would like to sell you some software, and links to his wares are sprinkled throughout.)