You don't. On all fix memory devices, if you want /need more user accessible memory, you BUY a device that has more. SD cards are external storage, not phone memory.
So in your case, moving everything you do not want on the phone off, then deleting/moving, all old text, email, attached pictures, video and music, to somewhere that is NOT the phone in question, best option an external hard drive, that allows you wired and WiFi access, even better if it allows internet access.
Only thing that can help going forward.... Delete you browser cache once a week. Same with all media files you don't plan on keeping, if you are keeping them, move them off the phone. Check your camera settings and REDUCE the file size to a lower mega pixel, rather than the max.