To quote the wiki:

The Meowmere has a 11.11% chance to be dropped by the Moon Lord boss.

To Summon the Moon lord you must:

1. Have defeated the Golem (and all preceding bosses).
2. Defeat the Lunatic Cultist outside of your maps dungeon.
3. Defeat all 4 of the Celestial Towers that appear.

OR

By using a Celestial Sigil, the boss summon item for the Moon Lord. (However this requires materials that are gained by beating the Celestial Towers, so arguably there's no benefit to spending them on re-summoning the boss, instead of crafting the end-game items they produce)

If you want to fight the Moon lord, there are a range of strategy guides available: http://terraria.gamepedia.com/Guide:Moon_Lord_strategies

Of course you can also get it from another player, if you're playing multiplayer. Or from an items map, if you're not bothered about earning everything legitimately.

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If you're on mobile Terraria:

Having looked at your other questions (and your other account) I guess you may be playing on mobile. If this is the case, you CANNOT get the Meowmere at this time. It's currently only available in the desktop version of the game.