How do you flash/root a galaxy s4 sph-l720t to work on another carrier besides sprint?!

[taotito]

I am switching from my iPhone to a galaxy s4 sph-l720t version that I was given. I found out that It is a sprint phone and I have T-Mobile. Is there a way to unlock it on T-Mobile? (or even Verizon since I might go to them next) I'm new to androids so you'll have to break it down for me please! and I've heard of flashing or rooting (is this the same thing?) would that help to do that and download some app?