If you are new to k-pop then you can decide which group appeals to you the most and then support it. Usually the first groups you get to know are Blackpink and BTS as they are one of the best known groups in the K-pop industry...

The begin of a k-pop fan.

IIn the beginning everything is new to you if you are an international fan then you usually have to learn the names of the idols first and sometimes even learn to tell the difference. For korean and asian fans this is easier because they already know habits.

Army or Blink?

Once you've found your way around the k-pop industry you can get to know several groups but some fans also stick with BTS or Blackpink and become very absorbed in it (which happens with other groups too) If you have become a real fan of BTS or Blackpink, you will be called an Army ( BTS fan) or a Blink ( Blackpink fan). These fandoms are the biggest fandoms in the k-pop industry. 

k-pop shippers

a k-pop shipper is a fan who links the idols together through chance accidents or encounters between the idols. Shippers get a lot of hate, which I also understand because it is usually by accident that the idols get into situations with each other. Sometimes idols are really a couple but prefer to keep it a secret. Most new k-pop fans sometimes ship the idols too, but once you become a real fan, most fans don't. Because of the shippers, the idols sometimes have to hold back at each other's concerts so that they are not shipped with each other.