Auto-Focus is the way to go. But if you really want to get good at portraits, you will eventually use AF to get you close and then fine-turn the focus manually.
Study LIGHT first. But you will also want to learn the basics of the 3 aspects of exposure you control with the camera (ISO/Aperture/Shutter speed).
A decent lens like a 50mm f/1.8 is inexpensive and a good place to start for portraits.