My friends and I are all fairly new to D&D. We've played about 5-8 sessions each. We decided to start an online campaign using roll20.net and I was chosen as the DM. I haven't been able to get my hands on a 5e DM's Guide, but I do have a PHB. Lately I've been trying to memorize all the rules of play (with the PHB) and designing the world in which our adventures will take place.
Do any, more experienced players have some advice for me? Do I need a DM's Guide? What mistakes can I avoid making? Basically, is there anything I should know before I start?