how to print string, if i input "1" then print "One", if i input "15" then print "fifteen", if i input "123" then print "one hundred and twenty-three "

If you want to "show" it in a Console so just use:

string Number = Console.readLine();
and then some if-questions like:

if(Number.Equals("1")){
Console.WriteLine("one")
}

if(Number.Equals("123")){
Console.WriteLine("onehundreed and twenty-three")
}

or with a switch case :D

i cant think of another possible Method

but deep in my brain theres a thought that there was a guy who hardcoded it ...
he added always read the first number like in 900 so its nine
and then he counted the numbers after that ... or so... if you google it im sure you will find it :D