EDIT:
I where provided several interfaces that describe all the functionality that the project should contain. I don’t know how to organize the project thow. this are my options I don’t know which one will be the most appropiate:
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Create a new project referencing the one that contains the interfaces. Start implementing the interfaces in that project.
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Place the implementation next to each interface in the project. In other words if the interface IFoo.cs is in the folder Model then place Foo.cs in the same folder.
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Place all interfaces in a folder called interfaces.
Edit 2
I basically where provided with something like:

the interfaces are not organized they are all in the same namespace and directory. I have to organize that and implement it using the mvvm pattern.
It depends on where you want to access/implement the interface.
a) If you want the interface to be at presentation tier then go-ahead and create a project called Common and place all your interface which you assumed to be use across multiple project.
b) If you want the interface to be visible on multiple tier such as Service contracts in WCF which should be available in both presentation tier and middle/service tier. Then you can put them in a Shared projects which will be shared among multiple tiers.