How do I script it so animations are in someones inventory?

1- You could create a button using the GUI system that plays an animation when a button is clicked. 2- You could create a tool that is placed in the user's inventory and causes the player to dance when the tool is "activated". You could even use the UserInputService that makes a player dance when a certain key is pressed. You would need to start off by creating the dance animation using Roblox's animation plugin (I suggest downloading a better plugin though because the new animation UI isn't as flexible as it's former self). You seem to be a beginner so I would suggest doing what most of us around here have done -- watch videos on YouTube, read the Roblox Wiki, dissect scripts written by other players, etc. People around here don't just hand you the solution for you to copy/paste. You're trying to accomplish something a bit more advanced but I know that you'll figure it out for sure!

Your going to want to make a tool, inside that tool make a LocalScript

Make sure to store animations in ServerStorage

Make sure to create a RemoteEvent in ReplicatedStorage

LocalScript inside Tool

script.Parent.Activated:connect(function()
    game.RepliactedStorage.Dance:FireServer("Dance Name") -- Fires Server Event and gives a Dance name
end)

Script inside ServerScriptService

game.RepliactedStorage.Dance.OnServerEvent:connect(function(player,dance)
    local animation = player.Character:LoadAnimation(game.ServerStorage[dance]) -- Loads a dance animation from given dance name from the Fired Event
    animation:Play()
end)