I use the following code to create Parallax Scrolling:
http://www.david-gouveia.com/portfolio/2d-camera-with-parallax-scrolling-in-xna/
Parallax Scrolling is working but I don't know how to add the player and the enemies. I tried to add a player class to the existing code, but if the player moves, then the camera isn't pointing at the player. The player leaves the camera viewport after a few seconds. 
I use the following code(as described in the tutorial), but it's not working:
 // Updates my camera to lock on the character
 _camera.LookAt(player.Playerposition);
What can I do so that the player is always the center of the camera?
How should I add the character and the enemies to the game? Should I create a layer for the character and the enemies? For example: 
new Layer(_camera) { Parallax = new Vector2(0.9f, 1.0f) }
At the moment, I don't use a layer for the player and I don't have implemented the enemies because I don't know how to do that.
My player class: 
public class Player
{
    Texture2D Playertex;
    public Vector2 Playerposition = new Vector2(400, 240);
    private Game1 game1;
    public Player(Game1 game)
    {
        game1 = game;
    }
    public void Load(ContentManager content)
    {
        Playertex = content.Load<Texture2D>("8bitmario");
        TouchPanel.EnabledGestures = GestureType.HorizontalDrag;
    }
    public void Update(GameTime gameTime)
    {
        while (TouchPanel.IsGestureAvailable)
        {
            GestureSample gs = TouchPanel.ReadGesture();
            switch (gs.GestureType)
            {
                case GestureType.HorizontalDrag:
                   Playerposition.X += 3f;                     
                break;
            }
        }
    }
    public void Render(SpriteBatch batch)
    {
        batch.Draw(Playertex, new Vector2(Playerposition.X - Playertex.Width / 2, Playerposition.Y - Playertex.Height / 2), Color.White);
    }
}
In Game1, I update the player and camera class:
    protected override void Update(GameTime gameTime)
    {
        // Updates my character's position
        player.Update(gameTime);
        // Updates my camera to lock on the character
        _camera.LookAt(player.Playerposition);
        base.Update(gameTime);
    }
    protected override void Draw(GameTime gameTime)
    {
        GraphicsDevice.Clear(Color.CornflowerBlue);
        foreach (Layer layer in _layers)
          layer.Draw(spriteBatch);
        spriteBatch.Begin(SpriteSortMode.Deferred, null, null, null, null, null, _camera.GetViewMatrix(new Vector2(0.0f, 0.0f)));
          player.Render(spriteBatch);
        spriteBatch.End();
        base.Draw(gameTime);
    }