hii i was developing my first app but get stuck on mainactivity.java

here is my code

Java Code:
package com.nterface;

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

public class MainActivity extends Activity {

    public void onCreate(Bundle savedInstanceState) {
        /** Called when the user clicks the Send button */
        public void sendMessage(View view) {
            // Do something in response to button
and here is the error that comes on line setContentView(R.layout.activity_main);}

Multiple markers at this line
- R cannot be resolved to a variable
- Line breakpoint:MainActivity [line: 12] -

help would be more appreciate