How to prompt user for location of file to reference

I'm working on a tool for work that I'm trying to make as simple as possible for everyone that will use it. I would like a macro that will prompt the user for the location of the file that contains reference data to go into the tool. When the user selects the reference data file, the macro should go in and import columns R:R, U:U, and AV:AV from the reference file into columns A:A, B:B, and C:C in the tool.

Thanks for your help. I'm extremely new to Macros, so please ask questions if I'm being unclear.

Ok, I can manually run the macro and it will give me the option to choose a file, and then it clears out my columns A:C, but it doesn't paste new data in them.

Also, when is this macros supposed to run? I would like it to run upon opening the document.

Thanks for the reply, making progress...