function should have a prototype

First of all, Turbo C++ is a very old and discontinued development environment which does not support the latest C programming language standards. You'd be better off using MinGW or Cygwin instead.

Secondly, the code pasted here is missing a few things; either you didn't copy it correctly, or the book you're using is not well-written. Here's what the code should look like using modern C:

#include <stdio.h>

int main( void )
{
  printf( "I see, I remember\n" );
}

As of the C99 language standard, implicit function declarations are no longer allowed; the compiler will no longer assume that a function returns int if the type isn't specified.

Also, you should not leave the parameter list for main empty; in this case, we're specifying that main takes no arguments using the void keyword¹.

printf writes to standard output, and standard output is usually line buffered, meaning the output won't appear on your console unless you fill the buffer or send a newline character ('\n').

Also as of C99, you don't need an explicit return from main, even though it's typed to return an integer value; by default, it will return 0 to the runtime environment when the program finishes executing.

Since you don't have the proper #include <stdio.h> your compiler thinks that printf is a function that should be declared somewhere, hence this:

function printf should have a prototype

also, you forgot ; at the end of the line

You are missing a semicolon(;) here. In C, every statement ends with a semicolon. You are also missing a newline (\n) at the end of the string to be printed. Your printf statement should look like this:

printf("I see, I remember\n");

And about the warning. Warnings are not any error, but are used to show that your code can show undefined behavior.

Use int main() as function name. Here, int is the return type of a function. int before main() means it means the function returns an integer.

So, you might wanna add return 0; at the end of your program as main function expects an int to return. The return 0; statement tells that your program has successfully completed the execution.

And also you are missing a header file #include<stdio.h> from which your printf() function definition comes. If you don't include header files, your compiler thinks that printf() is a function that should be declared somewhere.

Your code should look something like this:

#include<stdio.h>
int main()
{
    printf("I see, I remember\n");
    return 0;
}

I hope this clears your doubts.