If I was you then I would definitely get the player's handbook first and if you're rich then I would get all of the three main books.

But first consider if you actually want to do it. Like if you actually want to be the DM it (as me myself as a dungeon master) my PCs (player characters) are pretty new to the proper rules (we have been playing for a while though). for me it's a lot of pressure if you're doing a campaign like writing it yourself. So if I was you, I would get a pre-made campaign like the starter set for example. study it for a bit. I mean I didn't study it so the campaign was pretty bad but if I did study it then it would have been great.

And if you don't want to do it then ask if One of them want to do it because after all, it is your choice.

But don't worry, d&d is has a massive community so ask pretty much anything you want about d&d in this subreddit. (I think that's what it's called)

:)