How do I make a part that has a glowing selection part when you hover over it?

You can put a ClickDetector in the part. Change the CursorIcon property in the ClickDetector invisible/transparent. Then put a script in the ClickDetector. Use the MouseHoverEnter:() function in your script. Then put a SelectionBox in your part, and make the transparency of the SelectionBox 1. Then, in the script make it so once MouseHoverEnter:(), SelectionBox.Transparency = 0. Lastly, in the script make it so once MouseHoverLeave:(), SelectionBox.Transparency =1.

Your script should look like this:

script.Parent.ClickDetector.MouseHoverEnter:Connect(function()
    script.Parent.SelectionBox.Transparency = 0
end)

script.Parent.ClickDetector.MouseHoverLeave:Connect(function()
    script.Parent.SelectionBox.Transparency = 1
end)

You can use the SelectionBox object. You can either add it through code or put it in and enable/disable it when needed.

Make sure to set the Adornee to the Model or Part that should be outlined, otherwise nothing will appear.