There are two ways to do this:
1) You can temporary increase your mana points by drinking potions. This will only temporary give you extra mana points, but long enough to cast some spells and to gain \'practical experience\'. Note that you need the \'(light) blue potion\' (other colors might work also). Drinking \'mana potions\' won\'t work as they only increase your current amount of mana points, but never beyond its maximum.
2) You can permanently increase your mana points by training Charisma, Intelligence or Wisdom. This is extremely expensive though, and it might take more then one rank before you get only one additional mana point. (And you need several additional manapoints)