How to get selectitem in a combo box MVVM silverlight 5

See more:

C#

MVVM

Silverlight5

Hi, i need to set the value of a list of strings and get it from a selecteitem, this is my XAML

<ComboBox Name="ComboTipoPpa" Grid.Row="10" Grid.Column="1"
                      Height="26" Width="177" 
                      HorizontalAlignment="Left" 
                      Margin="10,1,0,1"
                      IsEnabled="{Binding ElementName=cbAgregarManual, Path=IsChecked}" 
                       ItemsSource="{Binding Path=ListTipoPpa, ValidatesOnNotifyDataErrors=False}" 
                      SelectedItem="{Binding Path=XXXXXX, Mode=TwoWay, NotifyOnValidationError=True}" />

the list in my mvvm (i'm using microsoft Prism)

public List<string> ListTipoPpa
{
    get
    {
        List<String> data = new List<string>();
        data.Add(ControlesResource.TextoTipoPPAConexion);
        data.Add(ControlesResource.TextoTipoPPAEjecutor);
        data.Add(ControlesResource.TextoTipoPPASTR);
        return data;
    }

}

** Note ** ControlesResource.TextoTipoPPAConexion this is from a resource file

What I have tried:

I think that I should do is this

public List<string> ListTipoPpa
{
    get
    {
        List<String> data = new List<string>();
        data.Add(ControlesResource.TextoTipoPPAConexion);
        data.Add(ControlesResource.TextoTipoPPAEjecutor);
        data.Add(ControlesResource.TextoTipoPPASTR);
        return data;
    }
    set
    {
        a_Variable_Defined_as_int = value;
            this.RaisePropertyChanged(() => this.ListTipoPpa);
    }
}

Any suggest