First model: 

public class FirstModel
{
	public string HelloWorld { get; set; }
	public FirstModel()
	{
		HelloWorld = "First model";
	}
}

Second model:

public class SecondModel
{
	public string HelloWorld { get; set; }

	public SecondModel()
	{
		HelloWorld = "Second model";
	}
}

XAML:

<Window x:Class="TwoDataContext.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:system="clr-namespace:System;assembly=mscorlib"
        xmlns:twoDataContext="clr-namespace:TwoDataContext"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Grid.Resources>
            <twoDataContext:FirstModel x:Key="FirstModel" />
            <twoDataContext:SecondModel x:Key="SecondModel" />
        </Grid.Resources>
        <TextBox DataContext = "{StaticResource FirstModel}" HorizontalAlignment="Left" Height="23" Margin="178,64,0,0" TextWrapping="Wrap" Text="{Binding HelloWorld}" VerticalAlignment="Top" Width="120"/>
        <TextBox DataContext = "{StaticResource SecondModel}" HorizontalAlignment="Left" Height="23" Margin="178,120,0,0" TextWrapping="Wrap" Text="{Binding HelloWorld}" VerticalAlignment="Top" Width="120"/>

    </Grid>
</Window>

Example you can download from here.

Let's first add RelayCommand class: 

class RelayCommand: ICommand
{
	private Action<object> _action;

	public RelayCommand(Action<object> action)
	{
		_action = action;
	}

	public bool CanExecute(object parameter)
	{
		return true;
	}

	public void Execute(object parameter)
	{
		if (parameter != null)
		{
			_action(parameter);
		}
		else
		{
			_action("Hello world");
		}
	}

	public event EventHandler CanExecuteChanged;
}

ICommand is an interface, and automatically you will receive methods which you have to implement. Now "click model":

public class MVVMButtonClickViewModel
{
	public MVVMButtonClickViewModel()
	{
		MVVMClick = new RelayCommand(new Action<object>(ShowMessage));
	}

	private ICommand m_ButtonCommand;

	public ICommand MVVMClick
	{
		get { return m_ButtonCommand; }
		set { m_ButtonCommand = value; }
	}

	public void ShowMessage(object obj)
	{
		MessageBox.Show("Test");
	}

}

and XAML:

<Window x:Class="MVVMButtonClick.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:MVVMButtonClick.ViewModel"
        Title="MainWindow" Height="350" Width="525">
    <Window.DataContext>
        <local:MVVMButtonClickViewModel />
    </Window.DataContext>
    <Grid>
        <Button Content="Button" HorizontalAlignment="Left" Margin="247,134,0,0" VerticalAlignment="Top" Width="75" Command="{Binding MVVMClick}"/>
    </Grid>
</Window>

As you can see, in button node, we have attribute like: Command="{Binding MVVMClick}, and of course don't forget to add DataContext, like: 

    <Window.DataContext>
        <local:MVVMButtonClickViewModel />
    </Window.DataContext>

Example you can download from here.

How to have button stretched over all window width: XAML:

<Window x:Class="DIexamples.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*" />
        </Grid.ColumnDefinitions>
        <Button Content="Button" Margin="0,23,0,0" VerticalAlignment="Top" />
        <Button Content="Button" Margin="0,0,0,0" VerticalAlignment="Top" />
    </Grid>
</Window>

Important lines to notice are:

<Grid.ColumnDefinitions>
	<ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>

And attribute: Margin="0,23,0,0"

Start new WPF application. Add TreeView control from toolbox.
Add one label.
My XAML looks like this:

<Window x:Class="MVVMtreeView.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <TreeView HorizontalAlignment="Left" Height="206" Margin="10,10,0,0" VerticalAlignment="Top" Width="497"/>
        <Label Content="Label" HorizontalAlignment="Left" Margin="10,284,0,0" VerticalAlignment="Top" RenderTransformOrigin="0.421,0.462" Width="497"/>

    </Grid>
</Window>

Then add model, I called it TreeViewModel, and it looks like this:

namespace MVVMtreeView
{
    public class TreeViewModel
    {
        public List<string> TreeViewModels { get; set; }

        public TreeViewModel()
        {
            TreeViewModels = new List<string>();
            TreeViewModels.Add("test");
        }
    }
}

In "code behind", in MainWindow.xaml.cs, in MainWindow method, add line DataContext = new TreeViewModel(); so, in my case MainWindow.xaml.cs looks like this:

namespace MVVMtreeView
{
    /// 
    /// Interaction logic for MainWindow.xaml
    /// 
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            DataContext = new TreeViewModel();
        }
    }
}

Now, bind tree view to a model, that means in the XAML (in my case MainWindow.xaml), add ItemsSource="{Binding Path=TreeViewModels}", I also add name to a tree view, then bind label content, so now my XAML looks like this:

<Window x:Class="MVVMtreeView.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <TreeView Name="tv" ItemsSource="{Binding Path=TreeViewModels}"  HorizontalAlignment="Left" Height="206" Margin="10,10,0,0" VerticalAlignment="Top" Width="497"/>
        <Label Content="{Binding ElementName=tv, Path=SelectedItem}" HorizontalAlignment="Left" Margin="10,284,0,0" VerticalAlignment="Top" RenderTransformOrigin="0.421,0.462" Width="497"/>

    </Grid>
</Window>

Example you can download from here.