How to implement an Android Service

Hi! I'm trying to periodically check with a web service if a new item has been added to a database. If there's a new item a listview that the user sees should be updated.
I'm using PCL and I have accomplish it creating a new Task with a timer inside. But this only works if the app is open. I want to do the same when the app is closed so the user gets a notification when a new item is added remotely.
I've been doing some research and I found andoid services, the info said that the service will continue, regardless of the app state, until you tell it to stop. But i haven't found many examples in how to implement it.

Here's the code that I had that works only when the app is opened:

 Task.Run(() =>
            {
                Device.StartTimer(TimeSpan.FromSeconds(secs), checkUpdates);
            });

  bool checkUpdates()
          {
            if (isOnline)
            {
                var lastUp= Application.Current.Properties["lastUpdate"] as string;

        //returns true if a new item is added
                if (service.newItem(DateTime.Parse(lastUp)))
                {
                    var itm = service.getNewItem(DateTime.Parse(lastUp));

                    Device.BeginInvokeOnMainThread(() =>
                    {
                            notification.ShowNotification(itm.Title, 1000);
                            listView.Insert(0, itm);
                     }                        
                    });                  
                }
                App.Current.Properties["lastUpdate"] = DateTime.Now.ToString();
            }
            return true;
        }

I'm trying to do the same with an Android service using dependency services, here's what I've got so far:

     public interface IService
        {
            void Start();
        }

      [Service]
        public class DependentService : Service, IService
        {
            public void Start()
            {
                var intent = new Intent(Android.App.Application.Context, typeof(DependentService));
                Android.App.Application.Context.StartService(intent);
            }

            public override IBinder OnBind(Intent intent)
            {
                return null;
            }

            public override StartCommandResult OnStartCommand(Intent intent, StartCommandFlags flags, int startId)
            {
                // From shared code or in your PCL


                return StartCommandResult.NotSticky;
            }
        }

The problem is that I don't know how to implement the code that I had in the timer to the service, can someone please help?