You can go into your spell-menu (where you learn the spells and empower them) and drag the spell-icon (the spell's picture) into your short-cut bar.
In the basic bar that you start with if you drag Magic Missile into the first blank-spot, you can use the spell after selecting a target by placing 1.
I hope this is all correct, been a while since I played.
Edit: You may also find the following links helpful:
General TipsGuides
Mage
FAQ