You can choose your class anytime, press "s" ingame to get up the skill tree, but remember to add the right one xD
The skill system is quite easy if you go for the european race, I wont suggest a new player to go for the chinese o.o
The build is how you build you character, press "C" everytime you lvl up and you will see that you got 3 points wich you ether can add on "int"(mp) or "str"(hp). How you do this will be your build, what you does here cant be changed! if you add int (wich i suggest on the second char xD) you get awsome skills wich kills pretty fast + that int incrases: Magical attack and magical defence. Str incrase physical attack and physical defense.
There are also hybrids (mix of both int and str) but to get them good you have to be very carefull to not ruin your char when you make an hybrid. I had one oce where I made it 69/70 and belive me it kicked ass!
You have to use your mind when you choose your build and skills.
When I see you mention the hunter, I guess you think on
wow right? The hunter, thief and trader arent classes, its different jobs you can take ^^ The trader makes money by transporting high priced stuff between the towns, the thief try to steal this and the hunter protect the traders for money and exp.
You can also change both jobs and "class" as often as you want (OFC! when you make your char, be sure for what build you go for

)
Feel free to pm me with more questions, or if I answered in a hard way, capishe?
and btw sorry for responding so late xD