To get the appActivity and appPackage use this command in adb shell:-
dumpsys window windows| grep -E 'mCurrentFocus|mFocusedApp'
and press Enter.
It will give you this kind of result :
mCurrentFocus=Window{421bddc0 u0 com.android.mms/com.android.mms.ui.ConversationList}
mFocusedApp=AppWindowToken{42171290 token=Token{420efc40 ActivityRecord{4210a3f0 u0 com.android.mms/.ui.ConversationList}}}
In these lines appActivity and appPackage is already given you just have to copy and use as i did.
Here is the code snippet which works for me. 
I am opening the message app on my intex aqua r2 device using this script.
import java.net.MalformedURLException;
import java.net.URL;
import org.openqa.selenium.remote.DesiredCapabilities;
import io.appium.java_client.android.AndroidDriver;
public class MessageApp
{
static AndroidDriver driver;
public static void main(String[] args) throws MalformedURLException
{
DesiredCapabilities capabilities = new DesiredCapabilities();
capabilities.setCapability("deviceName", "AQUAR2");
capabilities.setCapability("platformName", "Andriod");
capabilities.setCapability("platformVersion", "4.2.2");
capabilities.setCapability("appPackage", "com.android.mms");
capabilities.setCapability("appActivity", "com.android.mms.ui.ConversationList");
driver = new AndroidDriver(new URL("http://127.0.0.1:4738/wd/hub/"), capabilities);
}
}