I need Help

Congratulations on taking the dive! DM'ing can be very rewarding.

1. A pre-made module would probably be easiest and I'm guessing many here would recommend that. Personally, I find them restrictive and a bit harder to run and like to build my own stuff. I would say build a town, a couple adventure hooks, and some type of first encounter. If you have time you could flesh out each of the hooks, but you could also just have a general idea in mind and see what they bite on. From there you can slowly build the world outwards. For the town all you really need is a map, an idea where the key buildings are, a list of names to pick as needed, and a few interesting characters within the town. Make sure that you have some way to guide them towards the hooks, and you'll be off and running in no time. If that seems daunting, I'm willing to upload a map of mine of a town with some details, and you could then change or rebuild as you see fit.

2. DnD 5th edition is a much simpler version of 3.5 - which is what pathfinder is based on. There are some differences (advantage and disadvantage), but in general if you understand pathfinder, picking up 5e will be relatively simple. Just make sure you understand each classes skills that your players are using(those are different), read up on advantage and disadvantage and the basic combat section (it's really short) and you'll be gtg. There are still rules I look up or ask my players and that's okay.

3. There are some threads around here somewhere with tips for first time GMs. Just going from above I would say my biggest advice is to have fun. It sounds simple, but most first time GMs want to over complicate and stress about details. I would say don't do that. If you come upon a rule that you don't know, and can't look it up quickly just make a call and move on. I usually err on the side of the players but make it clear that we'll look the rule up later. Also for the first time player I would say keep it simple. Help them build the character and just walk them through the roles as needed - don't overwhelm right away. Also, if you're having fun with it - you'll be more descriptive, and naturally set the mood better.

Honestly most of the games boils down to this:

D20 roll + a proficiency bonus (if you have it) + appropriate ability modifier.

Make sure your first time player knows that and he's got 70% of the game down and can focus on just playing his character.

Congratulations on making the leap! DMing is great fun! If you approach it as a fun game, rather than a daunting responsibility, you'll have a better time.

To answer you questions:
1. Start with a starter set or pre-written adventure. Feel free to play it fast and loose with the "canon" of the story, but its good to have a starting point. Also, don't jump into a full on campaign for the first time. Start with a couple one-shots, or just a 2-3 session campaign/story. This will help ease the sting if things go wrong. Its easier to start over when you don't have months or years invested in a campaign/characters.

2. Pathfinder is simplified D&D 3.5. D&D 5e takes the best of all previous editions and pares it down to the essentials. It is also designed from the ground up to be easy to pick up for new and returning players. In short: I think you'll be fine.

3. Tips huh? Ok.

First, be prepared for the unexpected. Whatever that looks like for you. I always say, "Be prepared to improvise", but you can also prepare yourself by knowing your game setting well and having a couple random events/encounters ready at hand. Whatever works best for you. I realize you probably don't know what works best, so try things out. If your players throw you for a loop, try making something up on the fly. If it doesn't work, oh well. If you've followed my answer from question 1, it won't be hard to start over.

It is a good idea to have a conversation with a table in which you all set expectations. Make sure they understand that you are learning and experimenting, and encourage them to throw in tips/constructive criticism on the game. Setting reasonable expectations is one of the best ways to avoid future disappointment. If you can get your experienced players on board to assist you with the story and the rules, that will also go a long way!

Don't be afraid to fail! A great term I've heard thrown around in RPG circles is "Fail Forward". This is usually used in context of a game, where PCs fail, but the failure is used to help move the narrative forward, as opposed to halting the narrative. Here, I am using it in a real world context. Don't be afraid to screw up. Even something going "wrong" in the game can potentially end up being awesome!

I'll leave the hints with this one: Fake it 'till you make it! I've heard that Prince (you know, the artist formerly known as the Artist Formerly Known as Prince?) has a thing he does when he makes mistakes during a live show. He repeats the mistake, at least 3 times. Suddenly, the mistake sounds like it was on purpose and the audience is none the wiser. Remember, your players can't see behind your screen and they can't see into your head. If something gets messed up in the narrative, its possible to embrace it and act like it was your plan all along. I'm realizing as I write this, that I have practically based my entire GM style off doing this. Yikes!

First and foremost remember to have fun! That's really what everyone is there for in the end.

Good luck! Let us know how it goes!

I'm going to give Friar a hard time for a second, haha. I think Pathfinder is as complicated, if not more so, than 3.5 at this stage. I think thirty plus classes, some of which are classes combined together, and then there is the massive amount of third party stuff now. Anymore I like referring to Pathfinder as 3.75, haha.

I think that the Starter Set is a great way to begin your journey into being a DM in 5e. It is well done, and can fit into a homebrew world fairly easily. In fact, I think someone did that very thing and talked about it on the forums, but I can't remember who it was! If you change the names of the town, and use the people as guidelines rather than law you can make it your very own. Then there is all of the awesome info that Friar gave!