To a degree I agree with R-T, you might consider holding off on rooting until you find yourself with the need (that said, I tend to root my phones just days after I get them...).
To answer your question, most root apps come from the Play Store just like the other apps, they just specify 'root is required'.
Rooting the S4 isn't as easy as JBing an iPhone, but it's not really hard either. There is likely some terminology you need to get up to speed on (Google is your friend). And for this phone there are several steps that you MUST follow, but all of them are easy to do. You do want to make sure that you're in a quiet space with no interruptions while you do it, so that you can do each step in order as it's needed.
Also, before you begin the root process make sure your phone has a near-full charge. You won't actually need that much juice to root the phone, but if things go wrong and you find yourself struggling with a step in the process, that's where you'll appreciate having plenty of battery to work with.