The fastest way to gain experience in Minecraft is through killing hostile mobs. Hostile mobs are monsters that spawn in the Overworld such as zombies, spiders, and skeletons. Killing these mobs will give you experience points.
You can also gain experience by using furnaces to smelt ores, fishing, and even by trading with villagers. Another great way to gain experience is by killing Ender Dragons and completing the game.
Finally, you can gain experience by destroying experience orbs, which drop from blocks when they are broken. These orbs contain a small amount of experience, so it is best to save them and use them to level up.