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Your code is good, but there are a few improvements that you could make.

Determining what operation your user wants.

Instead of using if/else, I'd probably use a switch/case statement:

switch userInput
{
 case "+":
 Console.WriteLine(numberOne + numberTwo);
 break;
 case "-":
 Console.WriteLine(numberOne - numberTwo);
 break;
 case "*":
 Console.WriteLine(numberOne * numberTwo);
 break;
 case "/":
 Console.WriteLine(numberOne / numberTwo);
 break;
}

Error handling with user inputs

I'd probably also put a default clause, just in case your user doesn't enter one of + - * /:

switch userInput
{
 ...
 default:
 Console.WriteLine("Sorry, unknown operation.");
 break;
}

I would also use a try/catch block for figuring out what numberOne and numberTwo are:

try
{
 Console.WriteLine("Please select your first number: ");
 double numberOne = Convert.ToDouble(Console.ReadLine());
 
 Console.WriteLine("Please select your second number: ");
 double numberTwo = Convert.ToDouble(Console.ReadLine());
}
catch (FormatException)
{
 Console.WriteLine("Sorry, invalid number."); 
}

Your code is good, but there are a few improvements that you could make.

Determining what operation your user wants.

Instead of using if/else, I'd probably use a switch/case statement:

switch userInput
{
 case "+":
 Console.WriteLine(numberOne + numberTwo);
 break;
 case "-":
 Console.WriteLine(numberOne - numberTwo);
 break;
 case "*":
 Console.WriteLine(numberOne * numberTwo);
 break;
 case "/":
 Console.WriteLine(numberOne / numberTwo);
 break;
}

Error handling with user inputs

I'd probably also put a default clause, just in case your user doesn't enter one of + - * /:

switch userInput
{
 ...
 default:
 Console.WriteLine("Sorry, unknown operation.");
 break;
}

I would also use a try/catch block for figuring out what numberOne and numberTwo are:

try
{
 Console.WriteLine("Please select your first number: ");
 double numberOne = Convert.ToDouble(Console.ReadLine());
 
 Console.WriteLine("Please select your second number: ");
 double numberTwo = Convert.ToDouble(Console.ReadLine());
}
catch (FormatException)
{
 Console.WriteLine("Sorry, invalid number."); 
}

You might also want to check whether numberTwo is not zero when dividing:

switch userInput
{
 ...
 case "/":
 if numberTwo == 0
 {
 Console.WriteLine("Sorry, cannot divide by zero.");
 }
 else
 {
 Console.WriteLine(numberOne / numberTwo);
 }
 break;
}

Your code is good, but there are a few improvements that you could make.

Determining what operation your user wants.

Instead of using if/else, I'd probably use a switch/case statement:

switch userInput
{
 case "+":
 Console.WriteLine(numberOne + numberTwo);
 break;
 case "-":
 Console.WriteLine(numberOne - numberTwo);
 case "*":
 Console.WriteLine(numberOne * numberTwo);
 break;
 case "/":
 Console.WriteLine(numberOne / numberTwo);
}

Error handling with user inputs

I'd probably also put a default clause, just in case your user doesn't enter one of + - * /:

switch userInput
{
 ...
 default:
 Console.WriteLine("Sorry, unknown operation.");
 break;
}

I would also use a try/catch block for figuring out what numberOne and numberTwo are:

try
{
 Console.WriteLine("Please select your first number: ");
 double numberOne = Convert.ToDouble(Console.ReadLine());
 
 Console.WriteLine("Please select your second number: ");
 double numberTwo = Convert.ToDouble(Console.ReadLine());
}
catch (FormatException)
{
 Console.WriteLine("Sorry, invalid number."); 
}

Your code is good, but there are a few improvements that you could make.

Determining what operation your user wants.

Instead of using if/else, I'd probably use a switch/case statement:

switch userInput
{
 case "+":
 Console.WriteLine(numberOne + numberTwo);
 break;
 case "-":
 Console.WriteLine(numberOne - numberTwo);
 break;
 case "*":
 Console.WriteLine(numberOne * numberTwo);
 break;
 case "/":
 Console.WriteLine(numberOne / numberTwo);
 break;
}

Error handling with user inputs

I'd probably also put a default clause, just in case your user doesn't enter one of + - * /:

switch userInput
{
 ...
 default:
 Console.WriteLine("Sorry, unknown operation.");
 break;
}

I would also use a try/catch block for figuring out what numberOne and numberTwo are:

try
{
 Console.WriteLine("Please select your first number: ");
 double numberOne = Convert.ToDouble(Console.ReadLine());
 
 Console.WriteLine("Please select your second number: ");
 double numberTwo = Convert.ToDouble(Console.ReadLine());
}
catch (FormatException)
{
 Console.WriteLine("Sorry, invalid number."); 
}

Your code is good, but there are a few improvements that you could make.

Determining what operation your user wants.

Instead of using if/else, I'd probably use a switch/case statement:

switch userInput
{
 case "+":
 Console.WriteLine(numberOne + numberTwo);
 break;
 case "-":
 Console.WriteLine(numberOne - numberTwo);
 case "*":
 Console.WriteLine(numberOne * numberTwo);
 break;
 case "/":
 Console.WriteLine(numberOne / numberTwo);
}

Error handling with user inputs

I'd probably also put a default clause, just in case your user doesn't enter one of + - * /:

switch userInput
{
 ...
 default:
 Console.WriteLine("Sorry, unknown operation.");
 break;
}

I would also use a try/catch block for figuring out what numberOne and numberTwo are:

try
{
 Console.WriteLine("Please select your first number: ");
 double numberOne = Convert.ToDouble(Console.ReadLine());
 
 Console.WriteLine("Please select your second number: ");
 double numberTwo = Convert.ToDouble(Console.ReadLine());
}
catch (FormatException)
{
 // Whatever form of error checking you want
 Console.WriteLine("Sorry, invalid number."); 
}

Your code is good, but there are a few improvements that you could make.

Determining what operation your user wants.

Instead of using if/else, I'd probably use a switch/case statement:

switch userInput
{
 case "+":
 Console.WriteLine(numberOne + numberTwo);
 break;
 case "-":
 Console.WriteLine(numberOne - numberTwo);
 case "*":
 Console.WriteLine(numberOne * numberTwo);
 break;
 case "/":
 Console.WriteLine(numberOne / numberTwo);
}

Error handling with user inputs

I'd probably also put a default clause, just in case your user doesn't enter one of + - * /:

switch userInput
{
 ...
 default:
 Console.WriteLine("Sorry, unknown operation.");
 break;
}

I would also use a try/catch block for figuring out what numberOne and numberTwo are:

try
{
 Console.WriteLine("Please select your first number: ");
 double numberOne = Convert.ToDouble(Console.ReadLine());
 
 Console.WriteLine("Please select your second number: ");
 double numberTwo = Convert.ToDouble(Console.ReadLine());
}
catch (FormatException)
{
 Console.WriteLine("Sorry, invalid number."); 
}