There are several approaches to do the task.
I think that the simplest one is to reserve memory for the prefix "Today" in a character array and then after reading a string to copy this prefix in the beginning of the character array.
Here is a demonstrative program that uses this approach
#include <stdio.h>
#include <string.h>
#define N 100
int main(void)
{
char s[N];
char *prefix = "Today ";
const size_t PREFIX_LEN = strlen( prefix );
printf( "Enter a sentence: " );
fgets( s + PREFIX_LEN, N - PREFIX_LEN, stdin );
memcpy( s, prefix, PREFIX_LEN );
s[ strcspn( s, "\n") ] = '\0';
printf( "The sentence is: %s\n", s );
return 0;
}
The program output might look like
Enter a sentence: is a good day.
The sentence is: Today is a good day.
Take into account that the function
fgets includes the new line character in the array. This statement
s[ strcspn( s, "\n") ] = '\0';
removes it.