Regarding DMing on Roll20: If you're new, familiarize yourself with how tokens and the layers work, and how to transition between them. If you use fog of war/dynamic lighting/line of sight, [CTRL-L] when you have a player token selected is your friend in testing. Be careful of page size and organization: Large pages eat memory, lots of pages are hard to organize. You don't have to build everything if you have "location" maps to go with a low-detail different-scale "overland" map. Make game notes with text on the GM layer - that way you'll have descriptions right there visible to you but not the players. Use handouts to take notes, not just to make notes for players. Vary the terrain for player combats - let the environment become a character, and let different characters shine (your archer does well in open terrian, how does he do in a city or catacomb environment?) Be ready for players to question/investigate EVERY DETAIL you put in a map. So have a reason for each map choice, at least in your own head. If you have multiple pages planned for a session, have them grouped close so there is not a long delay moving the player ribbon through several screens.