my bike is a 3x9 XC bike, but I only use the one chainring, instead of all three. it is a shimano groupset, and I'm wondering what size cassette will be able to fit on my hub. I've looked around and someone said that a 1x11 will fit, but I just want to confirm that. any help is greatly appreciated, thank you!

You don't have to change anything on the rear.

I was planning on getting a new cassette just because I bought the bike used, and the old cassette has a bit of wear and tear

When in doubt, just replace the cassette (or any piece of equipment for that matter) with the exact same one, to avoid any incompatibility issues between 8,9,10,11,12 speed equipment. And it's common to replace your chain (in this case 9 speed) when you do get a new cassette (in this case 9 speed).

If it fits an 11 speed cassette you will also need new shifter, new derailleur and a narrow/wide chainring.

You can go all the way to 12 speed if you want. What you can't do is run any cassette that starts with a 10 tooth gear but 11-whatever mountain bike cassettes aren't a problem.

Forgot to add you will also need a new chain. 11 speed chain in thinner than 9 speed chain. It will shift for crap.