I wasted $100 on the DroneStart online course. It was more frustrating than useful and more or less skipped some of the most important things, like reading airspace on sectional charts and decoding TAF and METAR weather reports. I really like the free Remote Pilot 101 videos, which makes me wish I had paid for their course instead. That said, you can go it on your own. In addition to free online videos I used two books: Gleim Aviatiation FAA Knowledge Test Prep, and Test Prep 2018: Study and Prepare, from ADA, which I would say is a must-have, since it also includes a copy of the actual supplement book that you will use in the actual test. Both also have online codes for free practice tests.
Good luck!