double x;
double y;
double angle;    // in radians, if you want degrees: convert to radians before use
const double movementSpeedPerSec = 10.0;
...
Update(GameTime gameTime)
{
	x += movementSpeedPerSec * cos(angle) * gameTime.ElapsedGameTime.TotalSeconds;
        y += movementSpeedPerSec * -sin(angle) * gameTime.ElapsedGameTime.TotalSeconds;
}