Before you deploy your Wizards, you need to determine which spells they know. The usual method for selecting spells is for each player to randomly generate spells for each Wizard.
When you chose your army, you should have selected which Lore of Magic each of your Wizards is going to use (if you haven't, shame on you. Go back, choose now and remember to do it properly for your next game). This will sometimes be one of the Lores of Battle Magic – the eight most commonly used sorcerous disciplines included in this rulebook, but many Wizards have race-specific spell lores to choose from as well, as described in the appropriate Warhammer army book. The Lore of Undeath is available to any Wizard.
The spells a Wizard knows are determined by rolling dice and consulting the Lore of Magic in question. This is done openly, so both you and your opponent will be aware of the spells each Wizard has generated. If your army includes more than one Wizard, you can choose the order in which you generate their spells.
Regardless of the Lore of Magic your Wizard has chosen to use, you'll notice that the spells are numbered between one and six. To randomly generate the Wizard's spells roll a D6 for each of his Wizard levels and consult the chosen lore to see what spells have been generated.
Ordinarily, each spell can only be known once in the same army. The only exceptions are where a spell is not generated randomly, such as:
If a model has no choice over which spell(s) it knows, either because it is fixed by the model's rules, or because if has 'bought' a specific spell as part of army selection.
If the army book or spell lore clearly states that a model can exchange another spell for the spell in question.
Otherwise, if you roll a spell twice (whether for the same Wizard or for a different Wizard in the army) you must normally replace the duplicate spell with another of your choice from the same Lore. If you cannot, because all the other spells have already been taken by other Wizards, for example, then this extra spell slot is lost.