1: Find Out What Your Players Want and Adjust Accordingly
The golden rule of GMing is this: Give your Players What they Want. Before the campaign begins, ask each of your players a series of questions regarding play style and campaign preference. Then adjust your plans as much as you can bear to meet these needs.
Your players want to be Super Powered Gods? Lower the difficulty. Your players want a gritty experience? Raise the difficulty. Your players want an open world? Give them an open world. Your players want great roleplaying? Encourage character development and shush people who aren't focused.
Here is the questionnaire that you should send out to your players. New Campaign Questionnaire. Make sure to fully absorb and analyze the results!
Bonus Points: Make adjustments in game. If a player has expressed interest in thievery, then draw up some thieving quests. If the players want more combat, then give them more combat.
2: Prepare 3 Random Encounters and 10 Random NPCs
Have a list of random encounters and NPCs ready before the game. These do not need to be fully developed - they are just something to quickly draw on when put on the spot. Sketch up two or three encounters that your players are likely to stumble into, be it a fight with a gang, an assassin come to kill a PC, or a random fight with an owlbear in the woods. Get the statblocks of each of these enemies and a general gimmick for the battleground (on rooftops, plenty of trees, overlooking a cliff...). This way you will be able to quickly and seamlessly move the game forward.
Similarly, generate a list of about 10 NPC names and choose one distinguishing feature about each (missing left hand, persistent cough, drunk). Next time you are put on the spot for a random NPC, you have something to pull from. The heroes want to go to a shop? The shopkeep is the first NPC on the list. The guards come to arrest the PCs? The captain is the next NPC. By having a simple list of names and attributes, you can make your players believe that they are in a real world inhabited by real people. If you respond quickly enough, you can even make them think you have planned for every contingency!
Bonus points: Prepare one shop, one tavern, and one guard in detail, as your heroes are likely to encounter one of these each session.
3: Know the Rules
As GM you must not only run the game, you must be the expert on it. Carefully read and digest all information that could be pertinent to the session. If you only memorize one section, make sure it is Combat. Combat is where play tends to get bogged down, and it is the most rules oriented. Make sure you understand actions, movement, and attacking.
Make sure that you always have the rules handy. For Pathfinder, I highly suggest d20pfsrd.com, as looking things up online is as simple as a search function. For the first few sessions keep combat simple, but feel free to quickly look up rules you don't know. For the rest of your time GMing, if you don't know a rule come up with something on the spot and look it up after the game.
Bonus points: Understand the grapple rules. Just kidding, that's impossible. Instead, fully understand the classes of your PCs.
4: Have a Plan (and Don't be Afraid to Railroad)
For all GMs there is a struggle: Railroad vs Sandbox. Railroading is subtly forcing the characters down a set story path. When done correctly they will never know it is happening, and it can allow for more intricate plots. Sandboxing is allowing your characters free roam over the world, and permitting their actions and choices to have a great effect on the story and setting. When done correctly the players feel liberated and gain a strong connection to the world.
Most GMs will tell you to run a Sandbox game. Most players will explain that they would like a Sandbox game. However, if you are a new GM, then don't be afraid to Railroad a little bit. Sandboxing requires a lot more work than Railroading, and you have enough on your plate already.
Bonus Points: Allow your players two or three game altering choices, and have at least a vague idea where each path goes.
5: Know the Resources
There are A LOT of resources out there. Plenty of people play your game, and 99% of them will help the new guy. Besides the forums, which you should frequent, there are random generators, map creators, character creators, art databases, campaign hosts, virtual tabletops, forums, and player finders. Here is a link to the Resources you should check out:
GM & Player Resources
Bonus Points: Make an account on the Paizo Message Boards, and ask any questions you have in either the Rules forum or the Advice Forum.
6: Utilize Game Time and Keep Combat Moving
For the most part, people come to games wanting to game. Keep players focused, limit irrelevant conversation and, if you are feeling particularly Machiavellian, ban electronics. Move the game forward by directly engaging players. New players will often be in a rut if the obvious path does not present itself. Always give your players something to do, but be ready when they do something else.
Similarly, keep combat moving. Get an initiative tracker and place it within easy view. Make sure people know if their turns are coming up so they can prepare ahead of time. Know the rules. If you don't know a specific rule, use your best judgement and look them up afterwards.
Bonus Points: If a player is not ready on his turn, delay him. It might seem a little mean at first, but the next time he will be ready and everybody will thank you for it.
Bonus Points II: Read up on 10 Ways to Speed up Gameplay here
7: Keep the World Real
It is vital that the players understand they are in a living breathing world. One bad habit of new players is to treat campaigns like a plaything. By performing ridiculous actions, killing random people, and referencing things his character couldn't possibly know, a player can take away from the game world - as long as you let him get away with it.
Make sure that players understand that there are consequences for actions. If they make some snide remark to a king, they will suffer the king's wrath. If they attempt and fail to steal something, they will go to jail. If they randomly murder somebody in the street, they will get a price on their head. Ban any in-game references to things like computers, cell phones, or electric guitars. Unless everybody at the table is okay with it, these actions will detract from the game and completely prevent good roleplaying.
Bonus Points: Reward good roleplaying. If a player takes the world seriously then make sure his character is rewarded. Spend more time on the players spending more energy and everybody will catch on.
8: Start at Level 1 and Play a Module
Homebrewing (creating your own material) is great, but for new GMs you will probably want to run something somebody else wrote. This will allow you the breathing room and time to learn the rules and basics of game mastering. When running other people's work, you have two options: Modules, short three or four session adventures, or Adventure Paths, immense adventures that take dozens of sessions to complete. Usually, you will want to start with Modules. And always start at Level 1.
What to play? If you want a fun one-session exploration of the combat system, then check out Level 1 of Jacob's Tower. It has virtually no roleplaying, but it is a great quick intro that covers plenty of concepts. Since it only takes one session to run, you should then be able to quickly jump into whatever you want to run afterwards.
Dragons are Above My Pay Grade is a great way to dive into mechanics and roleplaying both. It explores travel, combat, magic, survival, negotiation, NPC interactions, and many other facets of the game. Best of all, it's a level 1 module where the players get to face off against a giant red dragon! Who doesn't want to fight a dragon for their first session?
Bonus Points: Modify your Module! Feel free to make tweaks and changes to your module based on the results of the New Campaign Questionnaire. Rule Number One should not be forgotten even when running other people's creations.
9: Say Yes, or the Rule of Cool
Roleplaying games are improvisation with dice. If your players have a cool or creative idea, let them run with it. Never let rules or your own plan for the session prevent excellent play. Never say no to an action or attempt. You can say no to the intended result, but never the attempt.
That's not to say that your players should succeed just because they try something cool. If a Level 1 Paladin in full plate is trying to leap from vine to vine in a jungle, let him try - but make sure his failure is spectacular, interesting, and moves the session forward.
Bonus Points: Replace "Yes" with "Yes, and..." You succeed, and something unforeseen happens. Replace "No" with "Yes, but..." You attempt and fail, but something interesting happens.
10: Get Feedback
Rule 10 goes hand in hand with Rule 1. After every session get feedback from your players. Make sure your players are enjoying the game, and find out how you can improve it. Ask for high points and low points. Ask what they want to see more, and what they want to see less. Proper communication is vital to successful GMing, and you will be amazing at how much you improve by simply asking how.
Bonus Points: Send out an e-mail to get the feedback in writing. That way you can always look over what you have improved upon and what you consistently need to change.