New Post: Rectangle is an ambiguous reference error

xTile defines a type called Rectangle which conflicts with a Rectangle type defined in XNA. If you import both the namespaces Microsoft.Xna.Framework and xTile.Dimensions at the top of your code files (which contain the respective Rectangle types), then the compiler doesn't know which Rectangle you mean, unless you fully qualify it with the namespace.

So basically, instead of declaring your rectangle as

Rectangle myRectangle = new Rectangle(...);

use the full type qualifier xTile.Dimensions.Rectangle as in

xTile.Dimensions.Rectangle myRectangle = new xTile.Dimensions.Rectangle(...);

If you want to use the XNA one instead, you need to specify it as Microsoft.Xna.Framework.Rectangle.

I hope that helps. Beyond this, I think you should look up some C# tutorials.