Are you playing fifth edition? And if so, do you have a copy of the rule books? If not, there's a couple places you can get a free pdf online. You definately want to have a very good understanding of the player's manual and dungeon master's guide.
In my highly limited experience as a DM, I would say the most important things I learned are don't let any one part of the game drag too slow (such as players spending 10 minutes debating each action in combat), and start small. New players especially will be happy with a simple adventure that helps them learn the rules, and if you're a new DM, running something unambitious for a first game will give you room to experiment and find your style
Amusingly, I just came to this site from reading a website on how to DM better. If you want some helpful (and angrily-phrased) advice on DMing, I recommend visiting TheAngryGm. However, be warned: A lot of the stuff on here is pretty detailed, and might not be helpful, especially if this is your first time DMing.
Also, don't stress out about it! Everyone is coming here to have fun, and your players will work with you rather than against you.