How to structure my unit test?

I am new to unit test and wondering how to start testing. The application I am currently working on, does not have any unit test. It a winform application and I am only interested to test the data layer of this application.

Here is an example.

public interface ICalculateSomething
{
     SomeOutout1 CalculateSomething1(SomeInput1 input1);
     SomeOutout2 CalculateOSomething2(SomeInput2 input2);
}

 public class CalculateSomething : ICalculateSomething
{

    SomeOutout1 ICalculateSomething.CalculateSomething1(SomeInput1 input1)
    {
        SomeOutout1.Prop1 = calculateFromInput1(input1.Prop1, input1.Prop2);

        SomeOutout1.Prop3 = calculateFromInput2(input1.Prop3, input1.Prop4);

        return SomeOutout1;
    }

    SomeOutout2 ICalculateSomething.CalculateOSomething2(SomeInput2 input2)
    {
        SomeOutout2.Prop1 = calculateFromInput1(input2.Prop1, input2.Prop2);

        SomeOutout2.Prop3 = calculateFromInput2(input2.Prop3, input2.Prop4);

        return SomeOutout2;
    }
}

I would like to test these two methods in the CalculateSomething. Those methods implementation are long and complicated. How should I structure my test?