In trying to learn the Korean language, I've come across a few differences in Korean translation. Can someone please clarify which Korean text I should be learning from?

For ex:

The material I’m using to learn Korean says “thank you” is gamsahabnida. I believe this is the formal way of saying it. The informal way would be gamsa.

However…on a website teaching Korean, it says it’s Kamsahamnida.

I’m trying to learn the language of South Korea.

Any tips would be helpful. Thanks.