Most spell casting in Magic is done using a magic wand item. While holding a wand, left-click to cast the active spell. Wands that have more than one spell will have a "wand inventory" that you can use to quick-change spells and manage any number of spells in a single wand.
You create a wand with a Nether Star and Blaze Rod
Once you have a wand, you upgrade it by interacting with an enchanting table.