For reference, the address for the old Atlas Games forum, which is now a read-only archive, is forums.delphiforums.com/atlasgames . Many basic questions about Ars Magica may already be answered there.
Also, these links may be of interest:
Official Ars Magica 5th Ed webpage - atlas-games.com/arm5/index.php
5th Edition Errata - atlas-games.com/arm5/arm5errata.php
Official Ars Magica 4th Ed webpage - atlas-games.com/arsmagica/index.php
4th Edition Errata - atlas-games.com/arsmagica/armerrata.php
Project Redcap - redcap.org/