Lots of good advice. I'm going to repeat some of what others have said as I add.
1) Start small and build up - in a way, I agree with [MENTION=6801558]robus[/MENTION] about Lost Mines of Phandelver. It might be good to read it and get ideas, but only run it if you feel comfortable with it. A lot of the times, I find it easier to plan my own smaller adventures, especially as a start. An initial encounter to get the PCs together and on a quest, with some exploration that includes 2-4 areas with some interactions and combats, is probably enough for 2-3 hours. Once that's done, see where everything stands, and build around it.
2) Watch how others play/run games. There are a ton of live streamed games out there. Don't get intimidated by the awesome DMs; just see how they run things. Here's a series that I found pretty helpful and simple to follow (
https://www.youtube.com/watch?v=jT3FRzEJDp8) but there are others too.
3) Get comfortable running monsters/NPCs and plan how they'll act before the game session. This is especially important for when you get into a situation where a "newbie" group gets into trouble with more powerful monsters/npcs. Often, even if monsters seem to have the upper hand, there are ways to change the situation to give the party a new chance. Sometimes, a new threat can surface and fight against the foes giving the PCs a chance to escape. Sometimes, one of the leaders of the monster/npc group could offer a deal to the PCs. Sometimes, a freak occurrence can frighten the monsters/npcs away.
4) When building encounters, don't make everyone a combat encounter. Give pcs a chance to use other skills/ideas to interact and explore. But, if you do make a combat encounter, always err on the "too easy" side rather than the "too hard" side. Give players a chance to warm up. If you do have a hard encounter, try to telegraph it to the players so that they know it might be dangerous.