Be careful when picking your class, as there are some serious differences between them. As a beginner, it's recommended you pick a powerful, straightforward class such as the Soldier or Barbarian, but feel free to experiment. Here's a quick rundown on the classes:
Knight: Very high armor, good attacks, slow speed
Elf: Good speed and magic, low armor and weak hand-to-hand
Barbarian: Awesome attack power, low armor and very low magic
Wizard: Excellent magic, good shot power, low armor and horrible hand-to-hand
Thief: Very high speed, but mediocre on other stats
Monk: Above average in most regards, but very weak shot power
Soldier: Nice combat ability and armor, low magic
Ninja: Awesome missile attacks and good speed, but no armor at all
Ranger: Perfectly balanced in all respects
Templar: Good armor and high magic, slow speed