How do I change a google map into something similar to the map in the 1st poster? How do I change it to blue and also include shadows?
You can do these steps in Photoshop:
These has a license that can be purchased if you need to use it commercially.
That will give you something like this:
At this stage you might want to consider extending/cloning edges and so forth depending on the usage.
This will result in something like this - not identical, but to show the steps in one approach to achieve this type of look:
Stamen Maps looks like a great resource for changing the styling of maps.