Resources
The Quizbowl Resource Center is the online forum for high school and collegiate quizbowlers nationwide. Everything from tournament announcements to theory to study strategies is discussed here.
Writing
There are lots of resources online for getting better at quizbowl. The best way to improve is to practice and write questions.
Jerry Vinokurov's How To Write Questions is probably the best single guide to writing questions. It explains both how to write, and what makes a question good. Read this if you write questions. (Also read a lot of questions.)
The Stanford Culture Guide is a great resource that lists the most important topics in pretty much every subject. Pick a list and start looking up things you haven't heard of.
Reading
The next best way to improve is to read questions.
ACFDB is an online database compiled by Carlo Angiuli which has over 10,000 tossups searchable by category, year, and difficulty. Start reading questions in a category that interests you, and look up things you've never heard of.
The Stanford Packet Archive is the largest repository of past tournaments on the Internet. Download a tournament and start reading through it. (You might want to start with "novice" and "junior bird" tournaments.)