There are few specific apps you should have if you want to stay in Korea.
Apps you already have on your phone, such as google maps, wouldn't work as it used to.
So, the first app you should have on your phone is 'Naver map'
please, please do think that you will be alright with your google map app.
because you are not!
Personally I though the same way, but I ended up downloading Naver app.
Second, download Ji Ha Cheol
This app is the most accurate subway app you can download. The app will tell you how many station you should go, and how long it will take to get your destination.
The last app you may need to download is 'Kakao Talk'
Kakao Talk is number 1 messenger application in Korea. Almost all Korean use Kakao talk and they share kakao talk ID when they need to share contact.
So, if you want to make friends in korea, and want to keep in touch even when you leave, you better have Kakao talk ID.