C :: Program To Get Series Of Integers From A User And Storing Those Values Into Array

The problem deals with writing a program to geta series of integers from a user and storing those values into an array. Then use a function called selection_sort, when given an array with n elements, the function must sort the array from smallest to largest values.

I have code, and im trying to get it to compile but im getting these implicit declaration errors and conflicting types. Here is my code and the compiler errors.

Code:
Compilation started at Sun Feb 10 20:14:48

gcc -Wall -o ex9-1 ex9-1.c
ex9-1.c: In function 'main':
ex9-1.c:16:5: warning: implicit declaration of function 'selection_sort' [-Wimplicit-function-declaration]
ex9-1.c:20:2: warning: implicit declaration of function 'prinf' [-Wimplicit-function-declaration]
ex9-1.c: At top level:

[Code] ...

Compilation exited abnormally with code 1 at Sun Feb 10 20:14:49

Code:
#include <stdio.h>
int main(void) {
int a[100], i, j, N;
printf("How many numbers will you be entering: ");
scanf("%d", &N);

[Code] .....