I would suggest leaving your catalog on the internal hard drive. If you have your images organized so that they are all within one master folder the process is very simple. All that is necessary is to COPY that master image folder to the new external hard drive. Then start Lightroom as you normally would. Go to the library module and right-click on that master folder and choose to update the folder location. In the dialog that appears, browse to the new external hard drive and choose that master folder. Click OK and you're done. After you have verified that all of the images are being found on the new external hard drive you can delete the folder on the internal hard drive. If the images are not all organized within a single folder then you will have to repeat this process for each folder. But it's just a simple process for each of those folders. I would suggest that you also store your catalog backups on the external hard drive, or get an additional external hard drive for storing backup files.