[@Skyswimsky] This is really solid advice, and if I were a new player, I would find this incredibly helpful. Kudos! :D

[@Ace of Jacks] Everyone's got a different style, but I think there's some fundamentals I think really help ensure a game's success.

Basically, if you get an idea, do not start an interest check until you are prepared. Imagine it like going for a job interview, you want to be prepared, do your research, sell yourself and your vision to people who have dozens of interest checks and running roleplays vying for their attention. You want to set yourself off on a strong footing, and that involves a lot of prep work, and like Sky said, sit on an idea while working on it for a couple weeks and see if it's still something that's strongly going to hold your interest. The hardest part of GMing is forcing yourself to keep the game moving forward when your inspiration and energy are tapped. 

A big way around that, I found, is asking players to have detailed histories for their characters and actually incorporating aspects of their personal stories in the main plot and doing similar things to really make the game feel like "our story" rather than "my story". After you have core players in a game, you'll want to do things so they feel invested in the game long-term, and that really is facilitated by letting them have some agency in creating their own side stories that run parallel to the plot. Let them make some NPCs, and talk with them to encourage and shape ideas; not every idea is going to work, but you can definitely help make something work. Players all have great ideas, it's just a matter of helping them shape them.

I always explain GMing to people like it's framing a house and players are the ones doing the drywall and painting. You build the foundations they can grow off of, but they're still following your structure and together, you make a house. The biggest bit of advice I can give is this; always keep ahead of your players and set and stick to deadlines. Try to think of an overall plot you want to follow, like your beginning, middle, and end pieces. think of NPCs, major plot points and events you expect to cover, and so on so forth. You can always change details as you go and the game changes, but always have those major pillars in mind. Think of your villains and the antagonists; a game needs some kind of conflict, and you want to make sure that whatever is the source of it is interesting.

Worldbuilding is really key in this area, and there's no such thing as too much on the GM side, but you want to give enough lore and info for players to reference and build characters off of without asking questions. Basically, think of yourself as a player, what would you in particular want to know if you were looking at the RP? For some lore that's not plot relevant but it helps flesh out governments, races, technology or whatever, I tend to do a codex section in hiders so people can look at it if they so choose for answers while the main bulk of the OOC gives the main information people would need to start the RP.

The most basic of story structures is who is your bad guy, what are they trying to do, and how are your heroes going to stop them? I find it really helps to occasionally do posts from the bad guy's POV for some plot reveals that the characters don't know about but players can anticipate in the future. You do want to have some surprises, but as a rule, it never hurts to keep your players informed and having enough of a mystery to keep them going. Think of how a novel goes, often chapters follow the bad guy's POV and part of the fun is the anticipation of where that information the reader knows comes to affect the hero. 

I kind of have this general guideline for when I make an RP or a character sheet where A happens because of B, resulting in C. For example, 

[i]Cindy decides to become a swordswoman because bandits killed her brother, and so she seeks out a mentor. The mentor takes her own because she reminds him of his own daughter, and she trains with him for years. Now a proficient warrior she travels the lands to protect villages, but this has drawn the ire of the bandit king. The bandit king has had his operations ruined because of Cindy, so he hires expert mercenaries to kill her. [/i]

It's a very simple format, but it gives justification for aspects of a character and the story and the resulting consequences, and those you can keep building off of indefinitely. And from there, you can put together a simple but effective plot pretty quickly. As a rule of thumb when it comes to roleplays, assume everything is going to take way longer than you expect. You can be stuck on the same fight scene for weeks and months if you're not careful because you tried to make it too complicated. You have to keep the plot moving, and that's why it's important to set deadlines for stuff. But going back to my devices for a plot, 

[i]The Bandit King has returned to reclaim the realm of his birth, and he's raised an army to conquer Fantasyland. Cindy lives in a village near the border, and her village is razed and her brother killed. Cindy spends years honing her skills and body and mind to fight the Bandit King, but she needs help. Cindy travels across Fantasyland recruiting capable adventurers, bringing fame and attention to her cause. Cindy and her friends fight off the Bandit King's minions, but they find something greater at stake when they rescue a mysterious princess. The mysterious princess was the Bandit King's bride, and she escaped him before his exile because of the power she wields. Cindy and friends now know if the princess is captured, her power could destroy any army and the Bandit King can rule the world. So Cindy and friends set out to find the mythical artifacts of the Kings and Queens of old that were ordained by the Gods, and they have to beat the Bandit King to them and his bride. With the artifacts in hand, Cindy and friends set out to confront the Bandit King one and for all. The Bandit King is defeated despite having his own artifacts, but now Cindy and her friends are now effectively demigods. What happens next?[/i]

Come up with your pillars for your plot and worry about fleshing out the first couple missions for your game and setting the stage, so to speak. And even when the game is going, make sure you take the time in the background to start working on the next part of the game; you don't want to run out of track before the train catches up.