The first level toilet makes a lot of noise after flushing, right as it is about finished filling up, lots of banging, and it's just getting worse as time goes on. After looking this up a bit it sounds like it is water hammer. A couple of things to consider-
1. I live in a two story condo that does not have a basement. Laundry and the hot water tank are on the second floor, second floor toilet is fine, as are all other faucets in the house, this is only happening with the toilet on the first floor.
2. It is really cold outside and therefore my water is shut down outside.
3. It's pretty much safe to say that I know basically nothing about plumbing.

Can someone please offer me some advice? I'm afraid that a pipe is going to burst, we're trying not to use this bathroom at all, but it's difficult to do that when we have guests over. Any advice you can provide is much appreciated. Thank you.

Do this:

1. Close and Open angle stop valve under toilet.

If this doesn't help, then...

2. You most likely have debris in fill valve located inside the tank. Depending what type of fill valve you have: remove upper portion of the fill valve, clean debris, and put it back together.

That should do...

OK, I just closed and then opened the angle stop under the toilet and that did not work. It's so loud that I'm afraid that I'm disturbing my neighbors, so I'll do it again (and your 2nd suggestion if it doesn't work).

One thing I did notice is that while the valve looked like it was super strong, it felt weaker than I expected, like I could snap it or something. Is this normal or could this be part of the problem? Thank you for your help!

Remove the led from the toilet tank see inside the tenk which type of the fill valve do you have, see the images below, thank.

John

The one on the right, in color, is what it looks like

Shut the water off under the tank and pop the lid. Unscrew the three screws on top of the fill valve and lift the float ball and stem assembly up and out. Pry out the washer and turn it over. Reseat it back firmly. Now hold your hand over the top of the open ballcock and have someone turn the water on o flush out the supply and ballcock body. Reassemble, making sure the small tube #5 is connected to the overflow tube, give it a flush. How's it sound now