You raise your shield in Ocarina of Time by pressing R. Standing still and pressing R makes Link crouch behind the shield for a full block, while pressing R during Z-targeting lets him keep the shield up and move at the same time. Those two modes cover almost every defensive situation in the game, and knowing which one you are in matters more than most players realise.
The Two Blocking Modes
Crouched block. Press R without targeting and Link drops into a crouch behind the shield. This is the widest coverage the game gives you, and it is what you want against projectiles coming from directly ahead.
Moving block. Press R while Z-targeting and Link holds the shield up while still circling, backing away, or sidestepping. This is the mode for actual combat, because standing still in a boss fight is rarely the right answer.
The Deku Shield and Fire
The Deku Shield is the first one you get, bought from the Kokiri Shop in Kokiri Forest for 40 rupees. It is made of wood, and that has a consequence that catches people out: expose it to flame and it burns instantly and is gone. Fire-based enemies will destroy it outright.
It can also be stolen if a Like Like swallows you. If that happens, you get it back by killing the Like Like before it escapes.
The Hylian Shield and Child Link
The Hylian Shield is found in a grave in the Kakariko Village Graveyard, or bought from the Bazaar for 80 rupees. It does not burn, which makes it a straight upgrade in most respects.
The exception is the one that surprises people. As child Link, the shield is too heavy to lift, so he carries it on his back even while defending. He goes through the blocking animation, but the shield is not in front of him. There is one clever use for that: it means child Link can stand under the falling debris on Death Mountain Trail and be protected by the shield on his back.
Adult Link uses it normally. If you are heading into difficult fights, our guide on how to beat Dark Link covers an encounter where shield timing genuinely decides the fight.
The Mirror Shield
The Mirror Shield is the last one you get, found in a chest in the adult section of the Spirit Temple after defeating an Iron Knuckle. Beyond blocking, its reflective face is a puzzle tool: it redirects light beams, and several rooms in that temple and beyond are built around that property.
If you are working through the dungeons in order, our Forest Temple guide covers the one that comes first for adult Link.
Frequently Asked Questions
Can you block every attack with a shield? No. Some heavy attacks break through a guard, and a few enemies attack from angles a shield does not cover.
What happens if the Deku Shield burns? It is destroyed permanently and you have to buy another from the Kokiri Shop.
Can adult Link use the Deku Shield? No. Adult Link is too large for it.
Do you need the Mirror Shield to finish the game? Yes. It is required to complete the Spirit Temple.