I'm just starting, so there are some basic things I don't know, and there is no major FAQ I can find, just the short one posted on every board.
OK, so anyone can learn anything from the trainers?
There must be some reason I'd want to start as a mage, since the warrior class has access to a lot more armor choices. What's the downside to making a plate mail wearing mage? And what about ranger?

You can learn any skill from any trainer, use any gear and put points into any attributes. The only reason select 'class' at the beginning is to give your character a baseline template from which to begin playing.

As to downsides? There's give and take. If you use high endurance "fighter" armor on a "mage" or "ranger" then your character will get the benefit of the extra endurance (even more so if you've taken "fighter" passive skills), but might be hampered by the negative movement percent typically assigned to such armor pieces. Also, since chest pieces typically give the highest bonuses, your "mage" might lose a ton of INT by selecting "fighter" armor.

lol i am just starting to but i still have questions about where some duengeons are