Assuming your variable is global and non static.

You need to declare it in a header file. We use extern keyword for this. As pointed out in comments, this keywords is not necessary, but most C users prefer to use it in headers, this is a convention.

stackoverflow.h:

#ifndef STACHOVERFLOW_H
#define STACHOVERFLOW_H

extern int my_var;

#ifndef

And you initialize it in source file. (Use of keyword extern is prohibited if you want to provide an initialization value).

stackoverflow.c

#include "stackoverflow.h"

int my_var = 50;

Do not put initialization value in a header, or you will get a linker error if the header is used at least twice.

Now you can use your variable in any other module by including the header.

main.c

#include <stdio.h>
#include "stackoverflow.h"

int main()
{
    printf("my_var = %d\n", my_var);
    return 0;
}

Including header "stackoverflow.h" in "stackoverflow.c" is a way to get sure definitions in source file match declarations in header file. This permit to have errors as soon as compilation instead of sometimes cryptic linker errors.

Edit: This is not at all the way to make a variable "private". You have to use a static variable to make it "private". See R Sahu's answer

/* a.h */
extern int globali;  /* Declaration for compilation */
/* Visible here */

Later make sure you define in (exactly) one of the compilation units.

/* something.c */
int globali = 42;  /* Definition for linking */