...
Intent intent = new Intent(TestApp.this, ShowSensors.class);
startActivity(intent);
...

package com.example.testapp;

public class ShowSensors extends Activity{
protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.sens);
}

    protected void onStart()
    {
        Context mContext = this.getApplicationContext();
        Toast mToast = Toast.makeText(mContext, "HALLO", Toast.LENGTH_LONG);
        mToast.show();
    }

}

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
      package="com.example.testapp"
      android:versionCode="1"
      android:versionName="1.0">

    
    <application android:icon="@drawable/icon" android:label="@string/app_name" android:debuggable="true">
        
        <activity android:name=".TestApp"
                  android:label="@string/app_name">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
                
            </intent-filter>
        </activity>
        <activity android:name = ".ShowSensors" android:label="@string/app_name">    
        </activity>
    </application>
</manifest>