In your activity layout activity_main.xml specify WebView component as following:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/activity_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

   <WebView
       android:id="@+id/webview"
       android:layout_width="match_parent"
       android:layout_height="match_parent">
   </WebView>
</RelativeLayout>

Then in your MainActivity.java get WebView component and load some URL:

package com.j2ko.webviewapp;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.webkit.WebView;

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        WebView webView = (WebView) findViewById(R.id.webview);
        webView.loadUrl("http://stackoverflow.com/");
    }
}

Also you need to specify INTERNET permission in your AndroidManifest.xml file otherwise WebView will not be able to load page:

<uses-permission android:name="android.permission.INTERNET"/>