Hello and welcome to Illarion!
I hope that I can answer your questions, feel free to ask anything else you want to know in addition. Magic in particular is a quite difficult topic but I'll try to give you an orientation what you can expect.
On constitution, the more constitution your character has, the less damage he'll take from "mudane" attacks with weapons etc. Also, hitpoints will regenerate faster and poison won't harm your character as much as it would harm a character with low constitution.
On mage stats, the default stats will work for a magician. Essence, willpower and intelligence are THE attributes for a mage. Willpower is the least important of those three, but that is a matter of "taste" - willpower helps more at magical defence than casting magic, but there, it also has a use.
A good mage must not have less than a total of 40 attribute points in the three mentioned attributes. The more, the better, but keep in mind that e.g. low strength will cause a hard time when you try to carry equipment.
On becoming a mage, currently, you have to apply at an ingame organisation that will assign a teacher to your character. Note that this system has its flaws and getting accepted as student might require some effort. See:
http://illarion.org/community/forums/vi ... hp?t=33608
Have fun!