If I were to go give two pieces of advice to new GMs, these would be those two pieces.
1. How to Make and Run NPCs
John Harper nailed it in half a paragraph in his new game Bootleggers, which I recommend you check out. It's pay what you want, and you should drop him some cash for it probably.
Give each NPC a desire and preferred method of action. What do they want? What will they do to get it? Describe two visual details about them (very tall, smokes a pipe. weather-beaten face, scruffy clothes. big eyes, walks with a limp).
Those two adjectives are all you need.
Re: the NPC's desire, following the advice of Apocalypse World, make it simple, obvious, and have the NPC focused on it absolutely. Better to go too far in this direction than not go far enough.
Giving NPCs a single, strong desire (a) invites an emotional/intellectual response, identifying, judging, what have you; and (b) it gives the PCs a hook. It's obvious what this person wants. Presumably there's some he reason he can't get it; and that's a service the PCs can probably provide in service of their own ends.
Which brings me to:
2. What to Say Now, and Next
If you don't know what to say, be a machine, and spit out a risk and/or reward.
Either give them the risk/reward itself or a lead to it.
The character of these will depend on the kind of thing we're playing, but, if we're playing the version of D&D where you get XP for killing things mostly, guess what you should do if you're at a GM-loss?
Give them things to kill. Either up in their face right now, or give them information on where things to kill might be lairing or having snacks.
If you're playing a game where you get XP for treasure, give them a glimpse of treasure, unguarded or not. Give them a treasure map. Give them a rumor.
Let's say you give them a rumor. "There's gold in the hills." So one of them says, "Let's go to the hills." The next thing you say should be a risk or reward. You are going to the hills when monsters. Run or fight. Or, you are going to the hills when you see a caravan loaded with goods (reward) beset by bandits (risk+reward).
Or let's say you're playing a game where they win by getting their character's dramatic issues resolved. So the next thing you say is either an opportunity to resolve that issue or something that stands in the way of that issue.
All your prep (including the assembling of modules, generators, tables, etc) is simply to give your brain a rest and to make sure you say things that are interesting and true to the game.