Hi everyone, I am new to the cycling world (just got my first road bike this year) and have a stupid question to ask. I bought a used bike and it looks like the cassette, chain, and probably crankset are due to be replaced. The problem is that I don't know anything about sizing these things!

The cassette is a Shimano HG50, 12-25t; the chain is a Shimano HG73, 9-speed; and the crankset is an FSA Gossamer MegaExo, 2-piece crankset.

I assume that the chain length would depend on the number of teeth on the cassette and crankset, but I'm really lost with all of this so any guidance would be really appreciated!
Thanks!