Hi jovceka,

Refer below sample.

Database

For this sample I have used of NorthWind database that you can download using the link given below.

Download Northwind Database

HTML

<script type="text/javascript" src="https://code.jquery.com/jquery-1.11.1.min.js"></script>
<link rel="stylesheet" type="text/css" href="https://cdn.datatables.net/1.10.7/css/jquery.dataTables.min.css" />
<script type="text/javascript" src="https://cdn.datatables.net/1.10.7/js/jquery.dataTables.min.js"></script>
<script type="text/javascript">
    $(function () {
        $('#tblCustomers').DataTable({
            "bServerSide": true,
            "bProcessing": true,
            "sPaginationType": "full_numbers",
            "pageLength": "5",
            "sAjaxSource": "WebService.asmx/GetData",
            "aoColumns": [
                            { "mData": "CustomerID" },
                            { "mData": "ContactName" },
                            { "mData": "City" },
                            { "mData": "Country" }
                        ],
            "fnServerData": function (source, data, callback) {
                $.ajax({
                    "dataType": 'json',
                    "contentType": "application/json; charset=utf-8",
                    "type": "GET",
                    "url": source,
                    "data": data,
                    "success": function (response) {
                        var json = jQuery.parseJSON(response.d);
                        callback(json);
                    }
                });
            }
        });
    });
</script>
<table id="tblCustomers">
    <thead>
        <tr>
            <th>CustomerID</th>
            <th>ContactName</th>
            <th>City</th>
            <th>Country</th>
        </tr>
    </thead>
</table>

WebServices

C#

using System.Linq;
using System.Web.Services;
using NorthwindModel;
using System.Text;
using System.Web.Script.Serialization;
using System.Web.Script.Services;

/// <summary>
/// Summary description for WebService
/// </summary>
[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
// To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line. 
[System.Web.Script.Services.ScriptService]
public class WebService : System.Web.Services.WebService
{
    [WebMethod]
    [ScriptMethod(UseHttpGet = true)]
    public string GetData(string sEcho, int iDisplayStart, int iDisplayLength)
    {
        NorthwindEntities entities = new NorthwindEntities();
        sSearch = sSearch == null ? "" : sSearch;
        int totalRecord = entities.Customers.Count();
        var result = (from customer in entities.Customers
                      select customer)
                    .OrderBy(customer => customer.CustomerID)
                    .Skip(iDisplayStart)
                    .Take(iDisplayLength).ToList();

        JavaScriptSerializer js = new JavaScriptSerializer();
        StringBuilder sb = new StringBuilder();
        sb.Clear();
        sb.Append("{");
        sb.Append("\"sEcho\": ");
        sb.Append(sEcho);
        sb.Append(",");
        sb.Append("\"iTotalRecords\": ");
        sb.Append(totalRecord);
        sb.Append(",");
        sb.Append("\"iTotalDisplayRecords\": ");
        sb.Append(totalRecord);
        sb.Append(",");
        sb.Append("\"aaData\": ");
        sb.Append(js.Serialize(result));
        sb.Append("}");
        return sb.ToString();
    }
}

VB.Net

Imports System.Web
Imports System.Web.Services
Imports System.Web.Services.Protocols

' To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line.
<System.Web.Script.Services.ScriptService()> _
<WebService(Namespace:="http://tempuri.org/")> _
<WebServiceBinding(ConformsTo:=WsiProfiles.BasicProfile1_1)> _
<Global.Microsoft.VisualBasic.CompilerServices.DesignerGenerated()> _
Public Class WebService
    <WebMethod()>
<ScriptMethod(UseHttpGet:=True)>
    Public Function GetData(ByVal sEcho As String, ByVal iDisplayStart As Integer, ByVal iDisplayLength As Integer) As String
        Dim entities As NorthwindEntities = New NorthwindEntities()
        sSearch = If(sSearch Is Nothing, "")
        Dim totalRecord As Integer = entities.Customers.Count()
        Dim result = (From customer In entities.Customers Select customer).OrderBy(Function(customer) customer.CustomerID).Skip(iDisplayStart).Take(iDisplayLength).ToList()
        Dim js As JavaScriptSerializer = New JavaScriptSerializer()
        Dim sb As StringBuilder = New StringBuilder()
        sb.Clear()
        sb.Append("{")
        sb.Append("""sEcho"": ")
        sb.Append(sEcho)
        sb.Append(",")
        sb.Append("""iTotalRecords"": ")
        sb.Append(totalRecord)
        sb.Append(",")
        sb.Append("""iTotalDisplayRecords"": ")
        sb.Append(totalRecord)
        sb.Append(",")
        sb.Append("""aaData"": ")
        sb.Append(js.Serialize(result))
        sb.Append("}")
        Return sb.ToString()
    End Function
End Class

Screenshot