What's the best way to find originals in itunes?

On my itunes, it is unable to find the original files of some of my songs. These are songs not purchased on itunes. The original files are on the other computer on the home network, that also has itunes. From what I understand, you can designate itunes where to look for files. How would I set this up? Is there a specific place it should look? Is there a way to make it easier to find? If they're shared files on a Network I have to figure out how to get everything together. Also I have an external hard drive, I'd like to save the songs on my computer to do a complete uninstall and install. What is the proper way to do this without losing the songs and is there something I could do at this time to alleviate the original file issue?