You are in Survival Mode. If you want something you will have to find/craft it.

If you want instant access to everything, you'll want to be in Creative Mode.

This applies to both versions of Minecraft.

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How do I switch Gamemodes?

Java, without recreating

1. Pause your game.
2. "Open to LAN"
3. In the settings, enable cheats.
4. Actually click the button that opens to LAN.
5. Type /gamemode creative.

Java, with recreating

1. When you create your world, select "Gamemode: Survival" and click until it shows "Gamemode: Creative".
2. Click on "More Options"
3. Ensure "Enable Cheats: On" is showing. If it is "Off", click the button to turn it on.

Bedrock

1. Go to Pause Menu
2. Go to World Settings
3. Enable Cheats
4. Say 'yes' to forfeiting the ability to earn any achievements
5. Turn on Creative in the same menu or open chat and type /gamemode creative.