• Right click on the solution, Add new project
• From the Test section, select an Unit Test Project
• Pick a name for the assembly - if you are testing project Foo, the name can be Foo.Tests
• Add a reference to the tested project in the unit test project references

MSTest (the default testing framework) requires you to have your test classes decorated by a [TestClass] attribute, and the test methods with a [TestMethod] attribute, and to be public.

[TestClass]
public class FizzBuzzFixture
{
    [TestMethod]
    public void Test1()
    {
        //arrange
        var solver = new FizzBuzzSolver();
        //act
        var result = solver.FizzBuzz(1);
        //assert
        Assert.AreEqual("1",result);
    }
}