How to get screen coordinate from geographic one?

I need to know, how can I convert some geographic coordinates to the screen one.

I want to remind you about desktop coordinates system, how it look like: I've developed soft, which is loading map fragments into matrix ( 10x10 ) in the fullscreen mode. The map images are loading perfectly.

But I have a task to draw some real elements on map, which do have longitude, latitude values.

So... I first of all think to convert geographic data to the web mecator values by using such a formulas:

internal Models.PointGeospatial ConvertFromMercatorToSpherical(double mercatorX, double mercatorY)
{
if (Math.Abs(mercatorX) < 180 && Math.Abs(mercatorY) < 90) return null;
if ((Math.Abs(mercatorX) > 20037508.3427892) || (Math.Abs(mercatorY) > 20037508.3427892)) return null;

double subitemFirst = mercatorX / 6378137.0;
double subitemSecond = subitemFirst * 57.295779513082323;
double subitemThird = Math.Floor((double)((subitemSecond + 180.0) / 360.0));
double subitemFourth = subitemSecond - (subitemThird * 360.0);
double subitemFifth = 1.5707963267948966 - (2.0 * Math.Atan(Math.Exp((-1.0 * mercatorY) / 6378137.0)));

var model = new Models.PointGeospatial()
{
Longitude = subitemFourth,
Latitude = subitemFifth * 57.295779513082323
};

return model;
}

internal Models.PointMercator ConvertFromSphericalToMercator(double longitude, double latitude)
{
if ((Math.Abs(longitude) > 180 || Math.Abs(latitude) > 90)) return null;

double subitemFirst = longitude * 0.017453292519943295;
double x = 6378137.0 * subitemFirst;
double subitemSecond = latitude * 0.017453292519943295;

var model = new Models.PointMercator()
{
X = 6378137.0 * subitemFirst,
Y = 3189068.5 * Math.Log((1.0 + Math.Sin(subitemSecond)) / (1.0 - Math.Sin(subitemSecond)))
};

return model;
}

Then I'm getting such values after conversion:

Building Id: 302

Geographic data: { Longitude: 37.5303305475 ; Latitude: 55.7481343179 }

Mercator data: { X: 4177857.2858509403; Y: 7508438.839224923 }

Really... I don't even imagine how could I place correctly the item on map just only knowing its longitude, latitude values and its mercator equivalent with having 640x480 opened window for e.g., on which I must place some pushpins...

The one detail I've foud, that first three digits from mercator values are not changing for the one city (and I understand they must not ), so the task to set a correct place for the item in the map with exact zoom level is rather diffcult for me and I don't understand how to solve this problem.

I'm listening for you suggestions, thanks!