Hope you are using appium server with Appium application.

Solution:

Open Appium.exe
Go to settings and click on it.
Select "Over ride existing sessions" and click on it.
Bingo you are done.

I am new to mobile testing - my only experience was to just test some apps in two or three real devices manually.

Since mobile development is agile, this needs to use test automation. I have experience in Selenium webdriver/grid and c# scripting and at least could use cross browser testing.

I am not sure about how cross-device/platform work in mobile testing. Do emulators work 100% accurate? Our users would be global and I know there will be language support as well. Any tips on mobile testing and automation in general? Thanks