How do I add a linear search for a customer name check...…meaning the system will say if it is a existing customer or a new customer (yes or no)

//This program is to Place the order of the number of items that you would like to rent for a special occasion
#include<stdio.h>

#include<stdlib.h>

#include <string.h>

#include <conio.h>

//Declaration Of Constants
int notents;
int nochairs;
int nostables;
int noltables;
//Struct
struct order {
    int numTents;
    int numChairs;
    int numSTables;
    int numLTables;
    int Orderdays;
    int Ordermonths;
    int CustomerID;
    double Totaldue;
    };
//Integer variables used in program
int numorder, choice, x = 0;

//Declaration of Functions
void menu();
void logo();
void order();
void exit();

struct order All[15];

int main(){
	logo();
    menu();
    return 0;
}


void logo(){
	printf("Welcome to Wendell`s T.T.C\n");
    printf("****************************\n");
}

void menu(){
    printf("Please choose an Option?\n");
    printf("1. Place an Order\n");
    printf("2. Exit\n");
    scanf("%d", & choice);
    switch (choice) {
    case 1:
        order();
        break;
    case 2:
        printf("Exiting");
        exit();
        break;

    default:
        printf("Invalid choice, Please try again\n ");
        menu();
        break;

    }
}

void order() {
	FILE * pt = fopen("NoItems.txt", "a+");
	printf("Price for Tents $100\n");
    printf("Price for Chairs $15\n");
    printf("Price for SmallTables $25\n");
    printf("Price for LargeTables $50\n\n");
    printf("How many orders would you like to place\n");
    scanf("%d", &numorder);
    int i;
    for (int i=1; i<=numorder; i++) {
    	printf("Current Order %d\n", i);
        printf("Number of Tents\t");
        scanf("%d",&All[i].numTents);
        printf("Number of Chairs\t");
        scanf("%d", &All[i].numChairs);
        printf("Number of Small tables\t");
        scanf("%d", &All[i].numSTables);
        printf("Number of large tables\t");
        scanf("%d", &All[i].numLTables);
        printf("Order month\t");
        scanf("%d", &All[i].Ordermonths);
        printf("Order day\t");
        scanf("%d", &All[i].Orderdays);
        All[i].Totaldue = (All[i].numTents * 100) + (All[i].numChairs * 15) + (All[i].numSTables * 25) + (All[i].numLTables * 50);
        printf("Total Due $%.2f \n",All[i].Totaldue);
		        fprintf(pt, "\n%d %d %d %d %d %d %lf ", All[i].numTents, All[i].numChairs, All[i].numSTables, All[i].numLTables, All[i].Orderdays, All[i].Ordermonths, All[i].Totaldue);
    
	//Closing  for loop
	}
    fclose(pt);
    menu();
}


void exit(){
	exit(0);
};