Mocking an implementation class with test class within unit test in C#

I'm trying to write a simple unit test where I have wrapped the HttpContext.Current.Server.MapPath with an interface and implementation. I'm not positive if the implementation is in the right place.

public class FooGenerator
{
    ServerPathProvider serverPathProvider = new ServerPathProvider();

    public string generateFoo()
    {
        BarWebService bws = new BarWebService(serverPathProvider.MapPath("~/path/file"));
        return stuff;
    }
}


public class ServerPathProvider : IPathProvider
{
    public string MapPath(string path)
    {
        return HttpContext.Current.Server.MapPath(path);
    }
}

In the test I have my own implementation I want to use, but I cannot figure out how to inject this into the real class during the unit test.

[TestClass()]
public class FooGeneratorTests
{
    [TestMethod()]
    public void generateFooTest()
    {
        FooGenerator fg = new FooGenerator();

        //Some kind of mock dependency injection
        Mock<IPathProvider> provider = new Mock<IPathProvider>();
        //stub ServerPathProvider object with provider mock

        string token = fg.generateFoo;
        Assert.IsNotNull(token);
    }
}

public class TestPathProvider : IPathProvider
{
    public string MapPath(string path)
    {
        return Path.Combine(@"C:\project\", path);
    }
}

Finally, here is my interface just in case. Basically I just want to swap out two implementations depending on whether or not I am unit test. This is my first unit test so much of this is new to me. Sorry if I'm missing something basic, but I've been digging through stack overflow for a while now and can't find the steps to do this part.

public interface IPathProvider
{
    string MapPath(string path);
}