Hello,
There is a lot of material available on various sites or sources, so you can find it, although it depends on what exactly you need to learn, because NVDA (and I think JAWS also) comes with an included user manual from which you can Learn more than the basics of a screen reader.
It all depends on what exactly you need and how you are willing to learn and what exactly you are going to learn. As I said above, ready-made tutorials, especially for NVDA, are enough to cover all aspects of it.
Later, when you finish the user manual, you can ask about anything through the mailing lists or here in the forum, or you can buy any material from the NVAccess website, they are really helpful as well or if you are using JAWS I am sure there will be materials for it as well, but there are more than a few sources for NVDA that can also be helpful in the learning process.