Error on my first android application. Hello world.

Hello, Im trying to do a simple Hello Word android application. But I'm having these errors.

Error(3,19)package android.app does not existError(4,18)package android.os does not existError(6,33) cannot find symbol
symbol:classActivityError(11,26)cannot find symbol
symbol:classBundleError(12,9)cannot find symbol
symbol  : variable superError(13,9)cannot find symbol
symbol  : method setContentView(int)Error(10,5)method does not override or implement a method from a supertype.

When I double click error, im get this:

package com.example.Hello;

import android.app.Activity;
import android.os.Bundle;

public class MyActivity extends Activity {
    /**     * Called when the activity is first created.     */    @Override    public void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.main);    }
}