Root doesn't give you more storage, it uses storage to save su and the app that controls it. I don't know where the "root and you get more storage" rumor came from.
If you need more app storage, add an SD card (up to 32GB on that phone). If you need more storage for particular apps to store data, that's up to how the apps were written - some can store data to the SD card and some can't. But rooting the phone won't give you more storage (and if you can't figure out how to get temporary root [which is what all these "rooting" apps do - get temporary root, install su, run any app that requires root and requests that su grant it], you shouldn't be fooling around inside the phone - you can turn it into a paper weight.