If you want a bunch of Bama advice on how to DM for the first time, that’s here: https://bamawing.com/fast-tips-on-dming/
If you want a deep dive on those first-timer tips, that’s here: https://bamawing.com/new-dm-yay/
If you are going to be DMing a large group, take a gander at this: https://bamawing.com/large-groups/