I'm going to end up repeating some stuff from page 1, but that's okay because it bears repeating.Professor Idle said:
However, I'm going to approach it differently, so... yay me. Ahem.
Okay, first off there are several different schools of thought about how to run a game. A lot of the advice you've gotten assumes that you will be using one school or another. Thus, some of the advice you've gotten conflicts. So let's talk game structure first.
Some D&D gamers prefer a "Sandbox" style game where the DM creates the world and then sits back and waits for the players to do stuff. The PCs will wander around, find quests (stuff you've intentionally prepared for them) but often they might go off on tangents and just explore.
Everyone telling you to avoid railroading, let the players do what they want, be who they want, etc - that's sandbox style.
Others prefer a more heavily narrative style. In narrative style, you take a greater degree of control over the story and the players will need to adapt some to fit into it. Certain character concepts might not be allowed (for instance, if you're running a game that takes place in an Eastern setting similar to China, you don't want a Celtic Brawler in there clashing with the setting (this happens more often in reverse, where PCs want to play ninjas in Europe, but the same idea applies).
Narrative style requires some amount of railroading so that the players know where to go, but still requires enough freedom that players have agency. The best way to do this is to provide plot points that act as road signs to tell the players where to go.
Many published adventure paths (like the Pathfinder adventure paths published by Paizo) function like this. Each part of the adventure has a link to the next part. Early on, the link tend to have "hooks" - the players learn that the foe they just defeated was working for someone in the nearby city, so if they want to get to the bottom of things, they'll have to follow up on that. Later adventures often rely on the characters being invested in the situation - they have to stop the Big Bad, so they will actively look for methods by which to do so.
And, finally, a GM can blend the two. A lot of first games end up being a blend due to DM inexperience or due to players trying to have one kind of game while the GM tries to have another. It takes time to get to know your players. Some players ONLY want a sandbox. Others ONLY want a narrative. Some (few) like both and can switch between style so long as they know which you intend to use.
Narrative requires some restrictions (to character concept and actions), but in return provides a story arc in addition to simply killing monsters.
Sandbox has few (or no) restrictions (aside from the rules), but can lack a sense of progress or accomplishment.
It is possible (and rewarding) to balance the two, but then you're entirely relying on the players to be curious enough about your storyline to pursue it on their own. And, if Skyrim has taught us anything, it's that a lot of players would rather wander aimlessly around Dwarven ruins than actually climb the mountain and talk to the monks. Or is that just me?
So there - a little long winded, but that's the best advice I can give: make sure both you and your players know what kind of game you are going to run, and make sure they understand how that will limit them.
Aside from that...
This has been said before, but avoid powerful ally NPCs. The PCs are the big damn heroes - don't let your temptation to include your self-insert Gandalf or Morganna or whomever derail that. Likewise, don't include any NPCs more plot-relevant than your PCs unless they are a McGuffin of some sort (ie, the PCs must rescue the kidnapped Prince). The PCs are the main characters of the story, even if there isn't a story (see narrative vs sandbox above).
Also, I cannot express enough how important it is to give your players one-on-one face time. Not everything has to be at the table. Sometimes taking someone aside for a private conversation can add some great spice to a game. Just make sure to be fair - don't let one player get all the asides, even if they're the party face.
Make sure to give your players the feeling of Agency - that they have an important impact on the world around them. Maybe it's just the illusion of agency, but so long as they don't know that, they'll feel like they're making a difference. Friendly NPCs that the PCs can help - and who are grateful to the PCs afterwards - is one good way to accomplish this.
And finally, don't be afraid to throw out hours of work because something happened in game. It's okay. There's a good chance you can still recycle much of it. And unexpected turns of events can make for very interesting storylines. If the PCs accidentally kill a major villain (or worse yet, an ally!) there are means of bringing them back. And I don't mean just back from the dead - one of my most infamous game villains was killed halfway through the campaign by the PCs. So he came back undead. And they killed him again. His soul sent minions to kidnap an important NPC and bring that NPC to him in the Abyss - the PCs had to go in and fight him in the Abyss itself.
On the other side of that, if a PC dies or turns evil, try to make an event out of it. An unexpected turn like that can hurt your plot, but you can also make it a memorable moment in your game by rolling with the punch rather than fighting it.
Good luck and good gaming to you.