A low-level wizard with a high Earth Magic skill is quite dangerous indeed, inflicting up to around 40 damage per hit against animals vulnerable to magic. A wizard may also want to practice defense for those times when he is unable to find anyone to group with and must fight solo. Wizards can only wear troll leather, regular leather, and cloth. This is reasonable, as they are usually weak and cannot find the energy to cast a spell and hold themselves up. Troll leather is the best armor for wizards, but it's expensive, so start saving.
Wizards typically use piercing weapons, so buy a sax or tiger's claw.
This is debateable. I would think that Dark Magic is most useful at low levels because it has so many offensive spells, and that's all people need when they first start out, anyway. For people who like to help others, the Heal spells in the Light Magic group would help. I myself prefer Earth Magic, for it represents the neutrality of my character, Xeus, and gives you Flame Orb, a spell which toasts ratlings and grey wolves at even level 1. Not enough people have gotten to a high enough level to judge what spell group is ultimately the best, since people haven't had the chance to mess with high level spells that much yet.