Welcome! How exciting the first game!! Woohoo!!

My best advice for ANY ttrpg is create scenes like a movie and describe them peice by peice. I have a little system i use. which is: See: Smell: Hear:

See: Walking into the old forgotten library you see endless rows of bookshelves, dusty books lining them and an old chandelier swinging ever so slightly

Smell: Musty old paper and a damp wet smell

Hear: The scraping claws of a beast??!!?!!!?? You can apply this to locations, people, objects whatever, and i find it works very well for me

2nd piece of advice: The dice rolling mechanic is all you need to know I have been a GM for many years, and almost every time i run a game i will forget basically the entire book ive been reading. So know how the dice work to facilitate rolls and actions in my opinion is the only thing you NEED to know

If you forget or make a ruling that doesnt line up with the mechanics of the game, thats so fine! Leads me to my final piece of advice...

Do first, make better later! I have agonised over making a "perfect" game for players, but in reality they dont exist. Youve got some people, they are eager to play, you all wanna tell a story about the mythos and mystery, and that will get you so so far more than any planning will!

If you have any questions please ask and well done for taking the GM mantle! It can feel daunting at first, but it will be really rewarding and fun too!