Nobody has any idea what they’re doing in the beginning.
New Players won’t know or care if you’re good or bad so stop judging yourself so harshly.
The Rulebooks are not infallible sacred scripture. They’re more like well thought out guidelines you can always choose to ignore.
Be 100% ready to throw out your precious idea when a player freely gives you a better one.
You aren’t under any sort of obligation to play with any person. If they’re ruining your fun talk to them about it. If differences are irreconcilable then for the love of the gods walk away from any toxic relationships.
Don’t save your best ideas for next session. There may not be a next session. Use everything you got every time you can and leave the table with as little regrets as possible.
You can steal ideas from anything and anywhere that gives you inspiration. If your references are obscure or unknown to your players they’ll think it was your idea in the first place. If your references are obvious they may have a rewarding “Aha!” moment as their brain draws links and comparisons. If your references are obvious and they’re wang-rods about it, see tip #5.
Worldbuilding may be fun but it may also be a colossal waste of time. It is a cardinal DM sin to write pages of backstory and genealogies for NPCs that the players will never interact with or care about. Every time you sit down to prep ask yourself “Will this come up in the session? Is this vital for them to know? Is there a way to tell them this naturally without a tedious info-dump?” Most of the things you think are uber important are answers to questions they never ask and artifacts that are irrelevant to the story of the PCs in the first place. Only prep what is necessary, and only time and practice can teach you what actually qualifies as necessary.
LISTEN TO YOUR PLAYERS. They will give you countless pieces of information about what they find fun and what they care about in the story. Focus on the fun, feed their hearts with what they desire. And then when you have them hooked, poke them. But not too much.
If your players want to do something NEVER just flatly say NO. If you must say NO because something is unreasonable in the world you are endeavoring to build you at the very least tell them WHY you are saying NO. Ideally, say “No…but you could…” and build on whatever suggested action they gave you. Eventually they will find the limits of what you find reasonable and plausible. If their view of reasonable and plausible is different than yours don’t be a wang-rod about it. Always try to find the middle ground, and sometimes (if they’re not being a wang-rod) just fucking give it to them.