finding subnet mask of a subnet number

So first off some background. Im day 1 into my 5 day net+ class. I have 0 networking, or even IT experience. Im trying to consume an ocean of information through what feels like a coffee stirrer

There are some basics i understand. i get binary. I know the default subnet masks for class a,b, and c but lets say im given a subnet number 192.168.1.0 and asked to find the subnet mask. Im lost. Ive been told its 255.255.255.240 but I dont know why or how to begin figuring it out if I was give some other subnet number.

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TLDR: explain it like Im 5, how do you find the subnet mask of a subnet number