Sort ObservableCollection on the basis of string

I am new to Xamarin and trying to sort ObservableCollection by using a string which I am getting on picker selection :

Model and ViewModel classes

public class TestListModel
{
public string FirstName { get; set; }
public string LastName { get; set; }
public string SSN { get; set; }
public string Gender { get; set; }
public string DOB { get; set; }
public string PhoneNumber { get; set; }
public string Address { get; set; }
public string State { get; set; }
public string City { get; set; }
}

public class TestViewModel
{
public ObservableCollection TestListModel { get; set; }

    public TestViewModel()
    {
    TestModel = new ObservableCollection<TestModel>
                {
                    new TestModel
                    {
                        FirstName = "John",
                        LastName ="Pasteka",
                        SSN="xxxxxxx",
                        Gender="Male",
                        DOB="02/09/1945",
                        PhoneNumber="xxxxxxx",
                        Address="xxxxxxx",
                        State="TX",
                        City="ROSENBERG"
                    },
                    new TestModel
                    {
                        FirstName = "peter",
                        LastName ="ETZEL",
                        SSN="xxxxxxx",
                        Gender="Male",
                        DOB="02/21/1945",
                        PhoneNumber="xxxxxxxxxxxxxx",
                        Address="xxxxxxxxxxx",
                        State="TX",
                        City="HOUSTON"
                    },
                    new TestModel
                    {
                        FirstName = "josh",
                        LastName ="STEVENS",
                        SSN="xxxxxxxxxxx",
                        Gender="Female",
                        DOB="01/05/1930",
                        PhoneNumber="xxxxxxxxxxx",
                        Address="xxxxxxxxxxx",
                        State="TX",
                        City="RICHMOND"
                    },
                    new TestModel
                    {
                        FirstName = "Rock",
                        LastName ="PARRA",
                        SSN="xxxxxxxxxxx",
                        Gender="Male",
                        DOB="10/09/2004",
                        PhoneNumber="xxxxxxxxxxx",
                        Address="xxxxxxxxxxx. ",
                        State="TX",
                        City="PATTISON"
                    },
                    new TestModel
                    {
                        FirstName = "aron",
                        LastName ="USMANI",
                        SSN="xxxxxxxxxxx",
                        Gender="Female",
                        DOB="08/22/1994",
                        PhoneNumber="xxxxxxxxxxx",
                        Address="xxxxxxxxxxx",
                        State="TX",
                        City="ROSENBERG"
                    }
                };
    }
}

I have a picker with values First Name, Last Name, Gender, SSN, Address.

private void Picker_OnSelectedIndexChanged(object sender, EventArgs e)
{
if (PickerCtl != null && PickerCtl.SelectedIndex <= PickerCtl.Items.Count)
{

    **Here I am getting filter selected by the user and i want to use this string to sort the items**

            var selecteditem = PickerCtl.Items[PickerCtl.SelectedIndex];
            var test = selecteditem.Split(' ');
            string newtest = "";
            if (test.Length > 1)
            {
                newtest = test[0] + test[1];
            }
            else 
            {
                newtest = test[0];
            }

    **Here I have tried the solution:**

            System.Reflection.PropertyInfo prop = typeof(string).GetProperty(newtest);

            TestViewModel Iitem = new TestViewModel();

            var result = Iitem.TestListModel.OrderBy(x => prop.GetValue(x, null));

    **After sorting I want to show the sorted result to the user in listview**

            lvSearch.ItemsSource = result;

        }
    }