1. Click to FileNewKotlin Activity.
  2. Choose a type of the Activity.
  3. Select name and other parameter for the Activity.
  4. Finish.

https://i.stack.imgur.com/kEBMA.png

Final class could look like this:

import android.support.v7.app.AppCompatActivity
import android.os.Bundle

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
    }
}