The ABC of tactics is the #1 classic in hattrick guides. Allthough the most recent update was already a while ago. This is the page to read when you’re starting a career as a hattrick manager. It’s available in several languages.

• Part 1: choice of team formations
• Part 1b: choice of formation, contd.
• Part 2a: the basics of player selection
• Part 2b: team selection using key values
• Part 3: Individual tactics
• Part 4: Tactical settings
• Appendix A: Formulas

Unfortunately the original site went down, but the content was secured by adding it to the Hattrick Wiki page.