This is a tutorial on how to use a SelectionBox and a few examples of it in use.
SelectionBoxes require a parent, like all objects and an Adornee. You can place the SelectionBox wherever you want as its parent, but you must set its adornee. An adornee what it shows up on, so basically it makes it overlay onto the part. Here's an example of how to create it and set its adornee:
For this, you just insert a LocalScript into StarterGui, which will be cloned to the Player's PlayerGui and when they bring their mouse over a part, it will have the SelectionBox over the part their mouse is over. It is also a good starting script that you could easily convert into a tool that does something, this is just a base script that can be expanded to anything you want.