When a function is defined to have a return value, then one needs to be returned of the correct type. You also have main defined as void return. Normally function main would return an int (as indeed your program does).

Code:
char FName[100];
...
printf("Please enter your first name:");
scanf("%s", FName);
What do you think would happen if the user enters a name that is more than 99 characters in length?