Step 1:
Open Android Studio

Step 2:
Select Flutter New Project option

Step 3:
Clear lib/main.dart and add followings

Using Simple code

                        import 'package:flutter/material.dart';
    
                        // the main() function is the starting point for every Flutter project
                        void main() {
                        
                          // calling this method (you guessed it) runs our app
                          runApp(
                          
                            Container(
                              color: Colors.green, // <-- change this
                            ),
                            
                          );
                        }

                       

Using class

                        import 'package:flutter/material.dart';

                        void main() {
                          runApp(const MyApp());
                        }
                        
                        class MyApp extends StatelessWidget {
                          const MyApp({Key? key}) : super(key: key);
                          @override
                          Widget build(BuildContext context) {
                            return const MaterialApp(
                              home: Text("Hello World Niluka"),
                            );
                          }
                        }

                        

App bar & body

We must use 'Scaffold layout' for creating page tree

                        import 'package:flutter/material.dart';

                        void main() {
                          runApp(const MyApp());
                        }

                        class MyApp extends StatelessWidget {
                          const MyApp({Key? key}) : super(key: key);

                          @override
                          Widget build(BuildContext context) {
                            return MaterialApp(
                              title: 'Welcome to Flutter',
                              home: Scaffold(
                                appBar: AppBar(
                                  title: const Text('Welcome to Flutter'),
                                ),
                                body: const Center(
                                  child: Text('Hello World'),
                                ),
                              ),
                            );
                          }
                        }

                      

1) appBar inside home
2) body inside home

Examples

Change home property of 'MaterialApp'

Container

                      home: Scaffold(
                        body: Container(
                          color: Colors.blue,
                          child: Center(
                            child: Text('Hello, Flutter!'),
                          ),
                        ),
                      )
                      

Column

                      Scaffold(
                          body: Column(
                            mainAxisAlignment: MainAxisAlignment.center,
                            children: [
                              Text('Item 1'),
                              Text('Item 2'),
                              Text('Item 3'),
                            ],
                          ),
                        )

                        

Row

                        Scaffold(
                            body: Row(
                              mainAxisAlignment: MainAxisAlignment.center,
                              children: [
                                Text('Item 1'),
                                Text('Item 2'),
                                Text('Item 3'),
                              ],
                            ),
                          )

                          

ListView

                          Scaffold(
                              body: ListView(
                                children: [
                                  ListTile(title: Text('Item 1')),
                                  ListTile(title: Text('Item 2')),
                                  ListTile(title: Text('Item 3')),
                                ],
                              ),
                            )
                            

Expanded

                            Scaffold(
                              body: Column(
                                children: [
                                  Text('Fixed Height Widget'),
                                  Expanded(
                                    child: Container(
                                      color: Colors.blue,
                                    ),
                                  ),
                                ],
                              ),
                            )

                            

Stack

                            Scaffold(
                              body: Stack(
                                children: [
                                  Positioned(
                                    top: 50,
                                    left: 50,
                                    child: Text('Overlay Text'),
                                  ),
                                  Center(
                                    child: Image.asset('assets/image.png'),
                                  ),
                                ],
                              ),
                            )