How to customize the Displayed text of a combbox

First of all you need a model class for customer:
Customer.cs

namespace WpfApplication
{
    class Customer
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public int Age { get; set; }
        public string Country { get; set; }
    }
}

For converting a customer object to "FirstName LastName"
you need a value converter class:
CustomerConverter.cs:

using System;
using System.Windows.Data;

namespace WpfApplication
{
    class CustomerConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            Customer customer = value as Customer;

            return string.Format("{0} {1}", customer.FirstName, customer.LastName);
        }

        public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }
}

MainWindow.xaml:
Here you define an object of the value converter
and define it as converter in the list box item template:

<Window x:Class="WpfApplication.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:WpfApplication"
        Title="MainWindow" Height="350" Width="525">
    
    <Window.Resources>
        <local:CustomerConverter x:Key="CustomerConv"></local:CustomerConverter>
    </Window.Resources>
    
    <Grid>
        <ComboBox ItemsSource="{Binding Customers}" Width="300" Height="30" Margin="10" VerticalAlignment="Top">
            <ComboBox.ItemTemplate>
                <DataTemplate>
                    <TextBlock Text="{Binding Converter={StaticResource CustomerConv}}"></TextBlock>
                </DataTemplate>
            </ComboBox.ItemTemplate>
        </ComboBox>
    </Grid>
</Window>

MainWindow.xaml.cs:

using System.Windows;

namespace WpfApplication
{
    public partial class MainWindow : Window
    {
        ViewModel vm;

        public MainWindow()
        {
            this.Loaded += MainWindow_Loaded;

            InitializeComponent();
        }

        void MainWindow_Loaded(object sender, RoutedEventArgs e)
        {
            vm = new ViewModel();

            this.DataContext = vm;
        }
    }
}

And of course you need a ViewModel.
ViewModel.cs

namespace WpfApplication
{
    class ViewModel : INotifyPropertyChanged
    {
        [AttributeUsageAttribute(AttributeTargets.Parameter, Inherited = false)]
        public sealed class CallerMemberNameAttribute : Attribute { }

        public event PropertyChangedEventHandler PropertyChanged = delegate { };

        public void OnPropertyChanged([CallerMemberName]string propertyName = null)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }
        }

        #region Constructors

        public ViewModel()
        {
            customers = new ObservableCollection<Customer>();

            Customers.Add(new Customer() { FirstName = "Bill", LastName = "Clinton", Country = "USA", Age = 60 });
            Customers.Add(new Customer() { FirstName = "Angela", LastName = "Merkel", Country = "Germany", Age = 60 });
            Customers.Add(new Customer() { FirstName = "Vladimir", LastName = "Putin", Country = "Russia", Age = 60 });
        }

        #endregion

        #region Fields

        ObservableCollection<Customer> customers;

        #endregion

        #region Properties

        public ObservableCollection<Customer> Customers
        {
            get
            {
                return customers;
            }
            set
            {
                if (customers != value)
                {
                    customers = value;

                    OnPropertyChanged();
                }
            }
        }

        #endregion
    }
}