for the +12% fire dmg: first youll need a weapon, skil with actual +fire dmg and then that fire dmg will increase in 12% lets say that you have 50 fire dmg and you had to it 50% fire dmg then youll have 50+25=75 fire dmg.
theres all kinds of dmg types at game including pierce, fire, ice, electricity, poisen etc.. and most of the monsters youll fight will have immunity to some dmg types and weakness to others
for the -3% life: that means that when ever the lich king attacks an enemy itll get instand -3% life and thats it, itll stay only -3%.