Ideally, you would use the MVVM design pattern to do this, but if you're looking for a simple and easy way to get this working, you could try this... Create UserControls to be the different subforms, and throw them into a frame. You could use a grid as well, but a frame allows you to use navigation (forward, back) if you need it.
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="200"/>
<ColumnDefinition Width="5"/>
<ColumnDefinition/>
</Grid.ColumnDefinitions>
<GridSplitter Grid.Column="1" VerticalAlignment="Stretch" HorizontalAlignment="Right"/>
<!--TreeView Code Here-->
<TreeView x:Name="treeView" SelectedItemChanged="TreeView_SelectedItemChanged">
<TreeViewItem Header="Devices" IsExpanded="True">
<TreeViewItem Header="Device 1" Tag="UserControl1.xaml"/>
<TreeViewItem Header="Device 2" Tag="UserControl2.xaml"/>
<TreeViewItem Header="Device 3" Tag="UserControl3.xaml"/>
</TreeViewItem>
<TreeViewItem Header="Users" IsExpanded="True">
<TreeViewItem Header="Add" Tag="UserControl4.xaml"/>
<TreeViewItem Header="Edit/Delete" Tag="UserControl5.xaml"/>
</TreeViewItem>
</TreeView>
<!--Frame to hold your subform (UserControl)-->
<Frame x:Name="SubForm" Grid.Column="2" NavigationUIVisibility="Hidden"/>
</Grid>
CodeBehind:
private void TreeView_SelectedItemChanged(object sender, RoutedPropertyChangedEventArgs<object> e)
{
TreeViewItem item = (TreeViewItem)treeView.SelectedItem;
SubForm.Source = new Uri(item.Tag.ToString(), UriKind.RelativeOrAbsolute);
}