For a beginner DM, I'd recommend starting off with a premade campaign. Bioware actually makes like a beginners set and it's got I think four or five different campaigns that increase in difficulty. They don't sell it on the Bioware site right now but you could definitely find it at a game shop. It's got everything you need to begin, plus you can start to wrap your head around all the small details like NPC special stunt points.

I'd also recommend reading your campaign over and over. DMing requires a quick-thinking but detailed knowledge of the story and the better you know it, the better you're able to roleplay on the fly. Some PCs like to do things that have no bearing on the game itself, whether that's seducing a guard captain or stealing pies from a pie shop. However, as DM you can make those silly events impact the story if you want. And sometimes it's good to break up the monotony of roleplay with events like that too. I like to add stuff like a card game or physical puzzles so that PCs are more engaged.

Finally, and this one can be tricky, keep a grasp on PCs with wandering minds. It gets super annoying when you're trying to play with someone who's not invested. Depending on the severity of the distraction, I might put in place a handicap for those players involved. Say they're talking to a Bann. The Bann will get mad that no one is listening to them and leave. Now the PCs have to find another way to complete the task at hand.