I wasn't aware that you could take the test online. I just started studying for the Technician
License.
I bought the spiral bound ARRL License Manual, 4th edition that the test is based on and have been following Dave Casler's YouTube channel. He walks you through the entire manual, giving really clear explanations of all the technical stuff.
If you want to dive in deep, his videos cover just about all ham topics including
equipment choices and how to evaluate what you need. Short of a personal mentor, I don't know how else you'd get up to speed. I am impressed with the
depth and volume of material he offers.
https://www.youtube.com/user/davecasler
Any recommendations on where to find practice tests?