Help! Explain the train system? (Seoul)

Hello!

I am living in Cheonan for the next 6 months. I am from a VERY small town in the United States and have zero experience in public transportation. I downloaded the app for the Subway system, but am having trouble understanding it. Some very basic questions...:)

-What time do the trains leave?
-Is anything in English at the train stations?
-How much do they cost?
-How do you know where you are at as to when you should get off?
-Is there a good way to know which train to use based on where I want to travel, i.e. if I wanted to visit Jeju Island or Seoul how do I get there?

I apologize for the very basic nature of the questions and really appreciate any help that anyone can offer!

And now I just realized I posted this on the wrong forum board, I'm sorry!

MacKenzie