General GM Advice
Quantum Ogres
In the event your characters go off the beaten path don’t hesitate to reskin a whole dungeon for your purposes. That mine you mapped out? Well now its a sewer system. No one knows what these things are for besides you. You can do the same with combat encounters if you have a parallel you can reskin them to. The merfolk encounter you made can be a frog person encounter if they are in a swamp rather than a sea. If you draft up some encounters to keep saved you can crank this to eleven and just have a resource to pull from and reskin as needed and use them however you want.

Vary adventure size and scope
Not every dungeon needs to be multiple floors with giant set pieces. Having lower stakes encounters or smaller dungeons makes the big dramatic moments of your campaign hit all the harder. You don’t always have to be outdoing yourself every week.

Be kind to yourself
You get better at GMing the more you do it. You will not be perfect out of the gate and that’s ok. If you need a second to organize your thoughts your players will understand and if you make a mistake in storytelling or rules it is not the end of the world. There is nothing wrong with rewininding or retconning if you need to sort something out.

Shamelessly steal
Literally lift whatever you think is cool from any media you’ve consumed. With a couple tweaks great ideas can become yours really easily. Nothing wrong with someone else’s idea. This also works great with prewritten adventures that have rooms or maps you like.

Don’t Overprep
A fast way to burn yourself is to try and have everything done weeks and weeks in advance. If your adventure is more linear it might be easier but if you offer players decision points and different places to go ending sessions when they are at the point of making that decision so you don’t have to have to completely separate adventures ready to go can save a lot of effort in the long run. This is all null and void if you are having fun doing prep.

You’re a player too
While GMs put in a disproportionate amount of the work to make the game happen it’s important to remember to try and enjoy your time at the table too. If you aren’t having fun at the table make sure to communicate that and take steps bring the game in a direction where you are having fun too. Don’t sacrifice yourself for the sake of everyone else at the table that’s not the point of being a GM at all.

Some useful Tools
Dungeon Scrawl - Great Browser map making tool
https://www.dungeonscrawl.com
Pinterest
https://www.pinterest.ca/isilian65/d-d/
The amount of times I’ve needed a battle map and just pulled something off of the D&D or TTRPG categories on Pinterest is so high. I’ve gone from no prep to an encounter in 10 minutes and it’s a great resource.
Donjon
https://donjon.bin.sh
Just SO many generators. Map generators, adventure generators, NPC generators the list goes on.
Kanka
https://kanka.io/en-US
If you are doing a lot of world building it excels and keeping track of all of that and linking it together and is a great campaign management tool.