If you're stuck on the demo mode after purchasing Minecraft, please visit this article.
If you would like to try Minecraft: Java Edition before deciding to purchase the game, you can play the demo mode by registering for an account, then downloading the Minecraft launcher. This version of the game lasts five in-game days, or about 100 minutes.
The demo mode is available to any Mojang account that is logged into the launcher but does not own a copy of Minecraft.
When you select Play Demo from the launcher, you'll be taken to the game's main screen with the option to start/continue a demo game, or reset the demo and start over from the beginning.
Playing the demo mode is a useful way to see if your system can run Minecraft. For more information, please also see the system requirements.