A shield can increase your character’s defense beyond the protection their armor provides. Your character must be wielding a shield in one hand to make use of it, and it grants its bonus to AC only if they use an action to Raise a Shield. This action grants the shield’s bonus to AC as a circumstance bonus until their next turn starts. A shield’s Speed penalty applies whenever your character is holding the shield, whether they have raised it or not.
Raise a Shield is the action most commonly used with shields. Most shields must be held in one hand, so you can’t hold anything with that hand and Raise a Shield. A buckler, however, doesn’t take up your hand, so you can Raise a Shield with a buckler if the hand is free (or, at the GM’s discretion, if it’s holding a simple, lightweight object that’s not a weapon). You lose the benefits of Raise a Shield if that hand is no longer free.
When you have a tower shield raised, you can use the Take Cover action (page 471) to increase the circumstance bonus to AC to +4. This lasts until the shield is no longer raised. If you would normally provide lesser cover against an attack, having your tower shield raised provides standard cover against it (and other creatures can Take Cover as normal using the cover from your shield).
If you have access to the Shield Block reaction (from your class or from a feat), you can use it while Raising your Shield to reduce the damage you take by an amount equal to the shield’s Hardness. Both you and the shield then take any remaining damage.