How do you place a object using your mouse?

I was just wondering how you would use your mouse to place a block on the floor according to where your mouse is.

You can only get the player's mouse from the client, or in a Local Script.

--// Local Script that Runs on the client
local player = game.Players.LocalPlayer
local mouse = plr:GetMouse()

There are tons of events for the mouse that you can read about on the wiki. One thing you'll probably want is the mouse target.

--// Local Script that Runs on the client
local player = game.Players.LocalPlayer
local mouse = plr:GetMouse()

local target

mouse.Button1Down:Connect(function()
	target = mouse.Target
	print(target.Name)
end)

You can get the CFrame of where the mouse it pointing in workspace using mouse.Hit.

--// Local Script that Runs on the client
local player = game.Players.LocalPlayer
local mouse = plr:GetMouse()

--// Makes a part where the mouse it pointing, when it clicks.
mouse.Button1Down:Connect(function()
	local part = Instance.new("Part")
	part.Parent = workspace
	part.CFrame = mouse.Hit
end)

That should be everything you need to attempt this problem yourself!

Come back if you have any questions!