C++ errors

[Zoma]

Time to once again over-help someone with an assignment

#include <iostream>
#include <cmath>
#include <string>

using namespace std;

string normalWords, rot;

void main()
{
cout << "Input the sentence you want translated:";
	string rot13 (string normalWords);
}

That's a bit strange. Not sure if you were trying to do,
but here's what I think you wanted to do:

#include <iostream>
#include <cmath>
#include <string>

using namespace std;

// This is a prototype.  You had yours inside the main program, which I'm not even
// sure is legal.
string rot13(string nomralWords);

void main()
{
 // Declare the variable names as local variables.
 // They were global before, meaning that they would
 // be overwritten in your function.  This is a bad
 // thing  
 string normalsWords, rot;

 cout << "Input the sentence you want translated:";

 // You need to get the stuff  
 // I'm guessing normalWords is where you wanted to
 // store the user input, and rot is where you
 // wanted to store the rotated input
 
 cin >> normalWords;
 rot = rot13(normalWords);
}

That should all be koshar, now. I'm not sure about the
type "string," as I stay away from most C++ types
If it doesn't work, I can help you use char[length] and char*

string rot13 (string normalWords)
{
	for (; 
	{
 		cin >> normalWords;
  		int i=0;
  		int over;
  		normalWords[i];
  
		if ((normalWords[i] >= 65) && (normalWords[i]<=90))
  		{
   			rot = normalWords[i] + 16;
   			if (rot > 90);
   			{
    				over = (rot - 90);
    				rot = (65-1) + over;
			}
			return rot;
	      }
	}
}

Ah, so that's where you read in normalWords. But a lot
Of that does not make sense. Why pass it in as a parameter
to the function if you're just going to read it in?
I'm guessing the function assumes that normalWords already has a value.
Also, you read in normalWords EVER iteration of the loop. But wait, the loop
returns after one character. Also, "normalWords[i];" does not make sense at
all. Maybe I'm wrong, but I don't think this will work.

My version:

string rot13(string someString)
{
	// We need a variable to use in this function
	string newString;

	// Loop through the string.
	// Not sure if strlen works on strings, so
	// you'll have to fix it if it doesn't
	for (int i = 0; i <= strlen(someString); i++)
	{
		// If the character is a valid letter, rotate it.
		// Just copy any other character
		if ((toupper(someString[i]) >= 'A') && (toupper(someString[i]) <= 'Z'))
  		{
			// If the letter is between A and M,
			// Add 13 to make it a letter between N and Z.
			// Otherwise, do the inverse.
			if (toupper(someString[i]) <= 13)
			{
				newString[i] = someString[i] + 13;
			}
			else
			{
				newString[i] = someString[i] - 13;
			}
		}
		else
		{
			newString[i] = someString[i];
		}
	}
	
	return newString;
}

Notice how I use two different variable names inside the function to avoid confusion.
I haven't tested this code, so I don't know if it will even compile. I'll
leave testing and fixing anything up to you