Sior |
1) This video is an hour long tutorial, a recorded Google Hangout session to train online GM's. It's specific for roll20.net utilization.
2) Personally, my own experience says "Just go for it". You'll learn as you go and pick up tips and tricks along the way. It takes a very little bit to get the image for the maps and the like to the proper scale and aligned to the grid, but once you have it, you have it.
More than that, one big piece of advice: Never be afraid to ask questions. We GM's have tips and tricks for almost everything. Some of us do things others don't and vice versa.
Good luck to ya! Hope to play at a table with you soon.