You need to learn the Linux boot process. (There are explanations and tutorials allover the web. During the boot process, various storage devices are mounted to different mount points. Basically what you want to do is either switch internal and external app storage or just mount the SD card as app storage and lose the internal storage. (If you have 1GB internal and a 128GB card, the extra GB is hardly worth even writing a line of code to keep.)
But it's not something I can give you a step by step on here for. Clear a small partition on a computer's drive, install Linux (Mint is probably the easiest for a Windows user) and, as you read, play with it, with the goal of mounting a USB SD card or memory stick as the main storage partition. If you mess up, all you have to do is pop the CD into the computer again, reinstall Linux, get back to just before you messed up and try it differently. You can't brick the computer, you can only brick Linux and you can reinstall it. (You could try it on the phone, but if you brick that, you'd better hope that you're set up to reflash the whole phone, and you've voided the warranty.)
Take it slow, read tutorials. And you'll be learning something new, which is always a good thing.