I am new for android and going to download new android SDK so which level should i choose 8 or newest 21?
you should download API 21 obviously and set Minimum SDK as per your requirement. That way you will be able to run your app from early Froyo(If you keep Minimum SDK as 8) phones to Lolipop ones.
So when you start with Android Development you will find configuration options in AndroidManifest.xml like MinimumSDK version(minimum version of android you want to support), TargetSDK(max version of OS u want to support) version and CompileSDK(usually equal to target SDk version) version.
Take a look here You should download API v21 and use min level 15 or 16 because versions below are already not so much distributed (API v10 = 7.4%, API v15 = 6.4%, API v16 = 18%).
来源：https://stackoverflow.com/questions/28294403/which-api-level-should-i-download-to-develop-an-android-app