I suggest you read the Documentation. It solves your question and more that could arise. This is exactly taken from the docs in the
map_to_world method: "Returns the local position of the top left corner of the cell corresponding to the given tilemap (grid-based) coordinates. To get the global position, use Node2D.toglobal"
In summary use
map_to_world(Vector(x,y)) (you need to pass it the tiles coordinates in a Vector2) to get the local position, and then use
to_global(your_local_position) to transform it into the global position.