How to expand and select first treeview item by default in treeview control in WPF

<TreeView Height="288" HorizontalAlignment="Left" Margin="18,23,0,0"  VerticalAlignment="Top" Width="89"
                  x:Name="tvCita" ItemsSource="{Binding CitaCol,Mode=TwoWay}" ItemTemplate="{StaticResource MasterTemplate}">
            <i:Interaction.Triggers>
                <i:EventTrigger EventName="SelectedItemChanged">
                    <cmd:EventToCommand Command="{Binding TreeviewItemClick}" CommandParameter="{Binding  SelectedItem, ElementName= tvCita, Mode=OneWay}"/>
                </i:EventTrigger>
            </i:Interaction.Triggers>
 </TreeView>
<DataTemplate x:Key="DetailTemplate">
            <TextBlock Text="{Binding Swift_File_Name}"/>
        </DataTemplate>
           
        <HierarchicalDataTemplate x:Key="MasterTemplate"
                                  ItemsSource="{ Binding SwiftCol,Mode=TwoWay}"
                                  ItemTemplate="{StaticResource DetailTemplate}">
            <TextBlock Text="{Binding CITA_Info_Id}"/>
        </HierarchicalDataTemplate>

I tried like this , but I am getting error

I created style under window.resource

<Style TargetType="TreeViewItem">      
<Setter Property="IsExpanded" Value="True" />  
</Style>  

I called this style in treeview control like this style="<staticResource TreeViewItem>

but i am getting runtime exception like this

'Set property 'System.Windows.FrameworkElement.Style' threw an exception.' Line number '40' and line position '79'.

How to fix it or How to expand and select first treeview item by default in treeview control in WPF