I have a Chroma 2 Case, but I need a key. How do I get one?
You can't earn a key. The only (somewhat legal) way to obtain a key is trough the steam market, buying one yourself (when you click the "Open" button, you'll be prompted to buy one). Another way is to receive it trough trade, many players trade skins for keys (basically turning skins into rl money)
TBH Another way to obtain a key is to sell your own skins (which is not advisable as cases never have anything good in them ...) What is my recommendation is to save the cases. Save every case you have and sell it in some time. I once started destroying cases as I tough they'd sell for 0.03$ which isn't that much and towards the last of my cases i noticed that they actually sell for like .50$ ...
You need to buy a key. Its fairly straight forward to buy one(just try to open a case). Now if you want to get keys without spending money you will need to go to other external websites and start betting or doing other stuff. I would not recommend doing this as many sites are shady. So if you really want to open the case just buy the case from inside the game.