Navigation between view using MVVM in WPF

Im new to WPF and MVVM.
Im trying to create Login window using MVVM and i succeeded to create.  
here is the Login.xmal code.

<Button x:Name="btnLogin" Content="Login" HorizontalAlignment="Left" Margin="51,0,0,10" 
                VerticalAlignment="Bottom" Width="124" Height="57" Grid.Column="1" 
                CommandParameter="{Binding ElementName=txtPassword}" 
                Command="{Binding LoginCommand}">           </Button><Button x:Name="btnClose" Content="Close" HorizontalAlignment="Left" Margin="180,0,0,10" 
        	VerticalAlignment="Bottom" Width="124" Height="57" Grid.Column="1"    Command="{Binding ExitCommand}"></Button><Label Content="User Name" Margin="10,74,0,0" VerticalAlignment="Top" Height="49" 
               VerticalContentAlignment="Center" Grid.Column="1" HorizontalAlignment="Left" Width="130"/><TextBox x:Name="txtUserName" HorizontalAlignment="Right" Height="49" Margin="0,74,10,0" 
                 TextWrapping="Wrap"  VerticalAlignment="Top" Width="185" 
                 VerticalContentAlignment="Center" Grid.Column="1" FontSize="18"><TextBox.Text><Binding Path="Username" Mode="OneWayToSource"><Binding.ValidationRules><ExceptionValidationRule></ExceptionValidationRule></Binding.ValidationRules></Binding></TextBox.Text></TextBox><Label Content="Password" Margin="10,128,0,0" VerticalAlignment="Top" Height="49" 
               VerticalContentAlignment="Center" Grid.Column="1" HorizontalAlignment="Left" Width="130"/><PasswordBox x:Name="txtPassword" HorizontalAlignment="Right"
                 Height="49" Margin="0,128,10,0"
        	  VerticalAlignment="Top" Width="185" 
        	VerticalContentAlignment="Center" Grid.Column="1" FontSize="18"></PasswordBox>

after this i have created the viewModeBase.cs class in which i implemented INotifyPropertyChanged and this included in LoginViewModel.cs...
here is LoginViewModel.cs code

 public class LoginViewModel : ViewModelBase
    {
        private string m_username;
        public string Username
        {
            get { return m_username; }
            set
            {
                m_username = value;
                OnPropertyChanged("Username");
            }
        }

        private string m_password;
        public string Password
        {
            get { return m_password; }
            set
            {
                m_password = value;
                OnPropertyChanged("Password");
            }
        }
        private DelegateCommand exitCommand;

        public ICommand ExitCommand
        {
            get
            {
                if (exitCommand == null)
                {
                    exitCommand =new DelegateCommand(Exit);
                }
                return exitCommand;
            }
        }

        private void Exit()
        {
            Application.Current.Shutdown();
        }

        public LoginViewModel()
        {

        }
        
        private DelegateCommand<object> loginCommand;
        public ICommand LoginCommand
        {
            get
            {
                if (loginCommand == null)
                {
                    loginCommand = new DelegateCommand<object>(Login);
                }
                return loginCommand;
            }
        }



        public void Login(object pPasswordBox)
        {
            try
            {
                if (string.IsNullOrEmpty(Username))
                {
                    MessageBox.Show("Username cannot be blank.");                    
                    return;
                }

                if (string.IsNullOrEmpty(((PasswordBox)pPasswordBox).Password))
                {
                    MessageBox.Show("Password cannot be blank.");                
                    return;
                }

                dlUsers odlUsers = new dlUsers();
                bool lResult = odlUsers.UserAuthentication(clsGymManagment.ConnectionString, Username, 
                    ((((PasswordBox)pPasswordBox).Password)));
                if (lResult)
                {
                    ///TODO: Need code to Hide Login Window and Open New XAML.....
                }
                else
                {
                    MessageBox.Show("Username/Password is wrong.");                   
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }         
    }

As i want to Hide LOGIN.XAML file and open UI.XAML file.. (UI.XAML you can consider any XAML window.)...
also it would be help full if you could assist me to navigation between Usercontrol on UI.XAML