For Wizards, Druids, and Clerics, hit 'K' to bring up the spellbook then click on the spells you want to prepare. You can prepare a number of spells equal to your level plus you spell-casting ability modifier. So, a 1st level Wizard with 16 Intelligence (i.e. a +3 modifier from their spell-casting ability) can prepare 4 spells. The other casters have 'spells known', so they don't need to prepare spells.
