Console program, let users to enter prices calculate subtotal, tax, and total with C Sharp
This isn't bad for a beginner, but there is much room for improvement.

Organization

The app is small enough that everything can fit inside Main, but a more seasoned developer will have Main be more barebones, with calls to other methods.

You would also want to separate out the UI, that is the prompting for input values, from the other Logic, that is the addition and display of values.

Variable Declaration and Naming

The good news is that your variables have some meaning in their names.  The bad news is you begin local variables with a capital letter (known as PascalCasing), when it should be camelCasing.  Thus, FirstOperand should be firstOperand, etc.

Also, CodeReview prefer to see each variable declared on its own line.  What I would expect you to end up with is something like:

decimal firstOperand;
decimal secondOperand;
decimal result;


Checking for Valid Input

You have no checks for valid input.  What happens if someone enters "dog" instead of 123.45?  You may want to read up on the Decimal.TryParse method to help out with that.

Things not needed

As mentioned, there is no need for Result and result since result is already a Decimal.

This line is too confusing:

Console.WriteLine("Addition Result: {0}", string.Format("{0}", result));

There is an overload for Console.WriteLine that accepts a string format.  This would simplify the line to:

Console.WriteLine("Addition Result: {0}", result);

The link you provide is for C# 2012.  Newer versions of C# support Interpolated Strings, prefaced with a $".  So you could also do the above with:

Console.WriteLine($"Addition Result: {result}");

If you wanted to control formatting of the numbers, it could be done one of 2 ways:

Console.WriteLine($"Addition Result: {result.ToString("0.00")}");

Or

Console.WriteLine($"Addition Result: {result:0.00}");

More Reading

To learn more, I found a decent post here on CR 'Welcome to Buzzway Subs'.  I recommend reading the original code, AND then follow up by reading the good advice in the answers.