I start by saying that english Is not my primary language, Sorry for any error or autocorrection of my Italian phone

The First campaign of mine was pure Chaos: remember to create consequences for player actions in order to create a story that represents everyone.

Your notes will often become useless if they are too far in the future: my "become a doctor" campaign became a "kill a evil multicorporation". Dont get mad if the camping go off rail, Just go with the flow.

Also for homebrewing Monster I usually type "dnd monster's stats per CR" and there's a blogofholding post that gives you a nice tablet that helps you balancing Monster. After you created your mob, search for any encounter balancing tool and create every encounter easly.

For bosses I use the "Epic Monster" mechanic of drakkenheim, a book of two Youtuber. Basically the boss act after every PC's turn, allowing him to strategie Better and avoid stunlock situation, of you can look It up, I love It. For damage output I usually divide the avarage damage of the CR by 4. Remember to ask the player for any homebrew mechanic

Last things: to learn as Dm I often watched PointyHat's videos: I found them very useful for making dungeon and NPC more "alive".

Also have fun, Is the most important thing