How to create a generic extension method?

I want to develop a Generic Extension Method which should arrange the string in alphabetical then by lengthwise ascending order.

I mean

string[] names = { "Jon", "Marc", "Joel",
                  "Thomas", "Copsey","Konrad","Andrew","Brian","Bill"};

var query = names.OrderBy(a => a.Length).ThenBy(a => a);

What is the way to develop Generic Extension Method?

I tried :

public static class ExtensionOperation
    {
        public static T[] AlphaLengthWise<T>(this T[] names)
        {
            var query = names.OrderBy(a => a.Length).ThenBy(a => a);
            return query;
        }
    }

I received :

Error 1: T does not contain definition for Length

Error 2: can not convert System.Linq.IOrderedEnumerable to T[].