Can I take my old mac and make it into an external hard drive

Can I take my old mac and make it into an external hard drive? It does not work anymore but I would like to make it into an external hard drive. I heard it is not to hard to do myself but I have important information I would like off of it and only really trust the apple store to do it. I know it is possible because I have had a friend that had it done after water went on her laptop. I would like to also know how much is this going to cost and if it is even worth getting done at the store or just do it myself.