Hi, so I'm a new DM and was wondering if there are any tips that you guys have? I'm starting off small, 3 close friends with a campaign that I heard was easy (Defiance of Phlan), if that gives you an idea as to where I'm at.

Best advice I can give is:

1. Expect the unexpected to happen.  You cant plan for everything because your players will pull things you never thought of. 

2. Fun trumps rules.  Basically if something breaks the rules or you can't remember the rule but it's fun and cool and fits...roll with it.  Just chalk it up to weirdness happens sometimes.

Good luck, have fun and dont sweat the small stuff.

Thank you!

Stick with the players handbook only for the first six months. If you don't have a dead pc at the end of the session don't worry about bad rule calls. Just tell the players you oops before the next session. I still making mistakes and I restarted in Sept 2016.

Congrats on starting out as a DM! It can be a lot of fun, especially just seeing your players have fun. I recommend checking out the first few videos of Matt Colville's "Running the Game" series on YouTube, as that can be a great intro for new DMs. 

I am not familiar with Defiance of Phlan, but know that your players will inevitably take it in a direction you weren't planning! When I first started DMing (did Curse of Strahd as my first campaign), I was a very "by the book" DM and it was uncomfortable for me if my players went outside what I read in the book. After a few sessions though, I got much more comfortable improving and now welcome the creativity players bring that take us to unexpected places. 

Besides all the other advice you will get I would say to not over think your game. If pirates attack the town and run off with the mayor you don't need to have a reason right away. A lot of the time your players will fill in the info for you. If the players are helping, knowing or not, they will begin feeling it's there story rather then just yours. Just have fun stuff happen, and see how it develops.

When you have time, you could try watching episodes of this series, which has the best DMing advice you will find on the internet all in one place:

If there is something you're not sure about, just make a snap choice. Don't stall the game to look something up or argue. Just make a judgement call and after the session is over look it up. Sometimes the rules align with your decision, sometimes they don't. You CAN choose to stick with your own call rather then the rules. That's why you're the GM.

Adding on to what Wysperra said about stalling the game... Try to be clear to your players. "I'm going to rule this way for now, and next session I'll come back with a more definitive answer going forward."

Everyone is nervous before sessions. Don't stress and you usually have a lot more fun. Don't think too much about not having a good accent or distinctive voices for the NPCs. (I suffer from this loss of accents) If the session sounds good, thought out, and fun, you're probably good and so is your session. 

Everyone else has given some great advice and I concur as well as second the idea that looking online for resources has helped me a lot. I will offer what I think has helped me the most.

I love to build a world and see the narrative and the story unfold!  Because I love this I want to see the players tell their stories and how it all weaves into the fabric of the entire campaign. Worldbuilding is awesome. Creating a story is awesome. Love being a DM and working with your players to have that story unfold and that has been extremely rewarding and fun for everyone. I like to think of it as the DM I am not the antagonist but instead the narrator.

Have fun out there!

Oh and if you're nervous before your session :

Welcome to the gloriously fun and frustrating job of being a DM.

Fun is the reason all of you, you and your players, get together. Remember it is supposed to be fun for you too. To ease your nervousness you need to be prepared. Read your adventure so you can be confident about where things are going. Be willing to make and fix mistakes. Let players do the same.

At the same time be willing to roll with the punches and go where the players lead you. Going off script WILL happen at some time. To help with that unease prepare a few things in advance that are not in the written adventure.

Prepare 3 NPCs. Just a name, a profession, a goal, and a personality quirk. If the PCs want to talk with random NPC #8 you have a few that you can fall back on.

Prepare 3 random encounters from the list of common monsters/encounters in the adventure you are using. This covers the times that the PCs are supposed to be talking/Role playing and the NPC needs to call for back-up because the PCs insult the NPC into combat.

Keep a list of house rules and game aids in a handy notebook, handout if on a virtual tabletop service such as Roll20. Also when it comes to edge cases and an interpretation of a ruling is subjective, mark off how it's interpreted by you (or by group consensus depending on your resolution) to keep things clear in the future.

If you're a fan of using random charts, tables, and the like for generating encounters, magic items, and such, you should roll it all up ahead of time. Some GMs can set things up on the spot, but for newcomer GMs this helps give a sense of what to plan for ahead of time. If your gaming group prefers all rolls to be in the open and you're doing a 'live game,' that may be harder to do than a VTT that can record such rolls. But given you are close friends with your players that shouldn't be an issue.

There's a lot of good free GM tools out there. Although not 5e specifically (it's OSR), Sine Nomine Publishing is notable for a bunch of system-neutral GMing aids. The Worlds Without Number has a free PDF full of advice and campaign generation tools for the fantasy genre.

Watch Matt Colville's Running The Game on youtube, it's a great source for information as a newer DM.  But the most important part is "Relax and have fun".

my advice: prep less, play more. overprepping kills your desire to play. While playing find the things that slow you down while dming. Prepare those things ahead. Stuff you're good at don't stress about. Prepare enough to get through the session. Over-prepping leads to dm burnout and heartbreak when the players Zag on you and all your prep is waster (prep is never wasted. you can just reuse it somewhere else)

Just let me tell you what it will feel like when you begin: Ever done Public Speaking or Ran presentations for clients?  It is a lot like that.  If you don’t have experience with that sort of thing you might be surprised and discomfited by how awkward and uncomfortable you feel with the attention glaring on you.  

It takes some getting use to.  Be prepared, stay focused and confident.   Speak clearly and enunciate.