General C++ advises about coding

I've recently stared learning C++ so Im new to all of it, Ive got my fourth "software" under development. Could someone have a look through and give me some constructive criticism on what should I change or keep the same? Any help will help :P Many Thanks

Code:

#include <iostream>
#include <string>
#include <stdlib.h>
#include <fstream>

using namespace std;


void mainmenu();


int choice; 
bool cinfail; 
int confirmation; 
string username, password, password2;

void writetofile(string username){
    ofstream writefile;
    string file = username+".txt";
    writefile.open(file.c_str());
    writefile << password;
    writefile.close();
    mainmenu(); }

void login(){
    cout << "You are being logged in!";}


void registerpassword(){
    cout << "Please enter the password:" << endl;
    cin >> password;
    cout << "Please renter your password:" << endl;
    cin >> password2;
    if (password == password2){
        cin.clear();
        cin.ignore(10000,'\n');
        writetofile(username);
        exit(1);
    }
    else;{
        cout << "Sorry invalid" << endl;
        registerpassword();
    }}


void registerme(){
    cout << "Please enter your username: " << endl;
    getline(cin, username);
    cout << "\nUsername -  \""<< username << "\"\nConfirm? \n\n[1] Yes\n[2] No" << endl;
    cin >> confirmation;
    if (confirmation == 1){
        registerpassword();
    }

    else; {
        cout << "Sorry invalid input, Please try again" << endl;
        cin.clear();
        cin.ignore(10000,'\n');
        registerme();
    }}


void exit(){
    exit(0);}

void mainmenu(){ cout << "Hello, Would you like to log in or register\n[1] Login\n[2] Register\n[3] Exit" <<endl; cin >> choice; do{
    cinfail = cin.fail();
    cin.clear();
    cin.ignore(10000,'\n');

    }while(cinfail == true);{
        switch(choice){
            case 1:
                login();
                break;

            case 2:
                registerme();
                break;

            case 3:
                exit();}}} 

main(){ 
mainmenu(); 
}