How do i get the mouse position? [SOLVED]

First of all, if there is an infinite loop without a form of a delay, it will crash your game. Change true into wait(). wait() is 0.299999999999 seconds long.

Gui's don't use "Position" like how parts do. They do something called "UDim2".

mainFrame.Position = UDim2.new(0.25, 0, 0.25, 0) --Changes "MainFrame's position to 0.25 scale to 0.25 scale."

Gui's position isn't close to Vector3.

The next part is mouseX. First we need to find mouse which is in a player. In 2010 they inserted a new method called :GetMouse() so, because this is a local script, we can use something called LocalPlayer. In order to do this we need to go into game.Players. So this is how we could get the mouse from LocalPlayer.

local player = game.Players.LocalPlayer --This is the player
local mouse = player:GetMouse() --This is the mouse.

The last part is the "X" part of mouseX. X is a property of mouse so we need a dot before it. It should look like

mouse.X

Now, with mouse.X we can only follow the X axis. We need to follow the Y axis also.

mouse.Y

Now if we add all this together we should get:

local mouse = game.Players.LocalPlayer:GetMouse()
while wait() do
    script.Parent.Position = UDim2.new(0, mouse.X,0, mouse.Y)
end

Let's say the thing that is following your gui is too big, so the mouse goes off your gui, just subtract a certain amount. For example

local mouse = game.Players.LocalPlayer:GetMouse()
while wait() do
    script.Parent.Position = UDim2.new(0, mouse.X-65,0, mouse.Y-65) --Notice the "-65"s. This could be any number.
end

I hope this helps!