mastery tree = heuskal pacheon bicheon ice fire lightning or force (each of those funny windows in skills menu)
X/300 --> X is ur mastery sum (every time u lvl up a mastery, X increases by 1) max is 300 atm
the character u have depends on your stats, if you go for pure str or str-hybrid wont be a bad one, wont do much damage (ice doesnt do as much damage as fire or light, but frosbites/freezes) but you would be able to tank a mob or a player for a while (high defense + frostbite you inflict) you pick
id recommend (if you like this game) is to start playing around with ur first character, to know how stuff works, and which skills u like more... then start again with the skills you like most

(it shouldnt take long, and imho is worth doin it)
ALL servers are ALWAYS full (xcept aege when i played there... i could join at first try, but then i was always laggin)