How do I open a second window from the first window in WPF?

When you have created a new WPF application you should have a .xaml file and a .cs file. These represent your main window. Create an additional .xaml file and .cs file to represent your sub window.

MainWindow.xaml

<Window x:Class="WpfApplication2.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>
        <Button Content="Open Window" Click="ButtonClicked" Height="25" HorizontalAlignment="Left" Margin="379,264,0,0" Name="button1" VerticalAlignment="Top" Width="100" />
    </Grid>
</Window>

MainWindow.xaml.cs

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
    }

    private void ButtonClicked(object sender, RoutedEventArgs e)
    {
        SubWindow subWindow = new SubWindow();
        subWindow.Show();
    }
}

Then add whatever additional code you need to these classes:

SubWindow.xaml
SubWindow.xaml.cs

private void button1_Click(object sender, RoutedEventArgs e)
{
    window2 win2 = new window2();
    win2.Show();
}

Write your code in window1.

private void Button_Click(object sender, RoutedEventArgs e)
{
    window2 win2 = new window2();
    win2.Show();
}