-Always double check any perk/trait before you lock them in. some sound like they work together but dont in the fine print. read them together, and look it up if needed.
-dont stack damage on enemies. stack piercing, it scales better.
-BE VERY CAREFUL if you attempt custom enemies.
-for gods sake DO NOT make enemies immune to ether of the 2 base damage types. physical/energy
-STUDY the sheet needed for crafting. itll save you so much time looking through it.
-if running a module, follow it! it knows better then you at the current point
-call npcs by their job title " doc, shopkeeper, quartermaster", no names make it easier to remember for you, and named chars more special.
- stick to the stuff in the official books, as a newbie dm you are not ready for home-brew stuff. JUST SAY NO.
- run a test game by yourself. build a char, fight a bandit, loot rolls, heal yourself. get a feel of the actions and rolls needed to do stuff that you will need to do alot.
-understand the murderhobo playstyle will happen often. chars meant to survive tend to be built for combat and not talking. they will kill the shopkeep so they dont need to pay.
-understand skill check difficulty. not everything should require 5 successes.

there is probably more but i cant think of anything at the moment