The amount of magicka you have is determined by several factors:
1. Intelligence - your base magicka is equal to twice your intelligence 2. Race - Bretons and Altmer receive bonuses to magicka 3. Birthsign - The Mage, The Apprentice, and The Atronach all provide magicka bonuses
For a character you've already created, your only real choices are:
A. increase your intelligence (at level up) B. make potions or spells that fortify magicka (temporarily)