C++头文件交互

好吧，所以我对C++还很陌生，我有几个关于头文件的问题。。。

1.）哪些变量应该声明，哪些不应该在头文件中声明？

2.）当你在头文件中声明一个变量时，你应该使用extern吗？

这是我的头文件：

#ifndef MAIN_H
#define MAIN_H
class Main
{
public:
        int main(); //Constructor
        virtual ~Main(); //Destructor
        double initialVelocity;
        double initialAngle;
private:
        double degToRad(double angle);
        void simulate(double angle, double velocity);
};
#endif

这是我的Main.cpp

/*******************************************************************
 * This program will take input for initial velocity (fps), and a launch angle
 * based on this information, the current posotion of the object thrown will be
 * calculated until it hits the ground.
 *
 * 
 * Date: 30 August 2013
 * Version 1.0
 *
**/
# include "Main.h"
# include <iostream>
# include <fstream>
# include <cmath>
using namespace std;
/******************************************************************
 * General Variables
**/
const int GRAVITY_FACTOR = -16;
const int GROUND = 0;
const double PI = atan(1.0)*4;
double initialVelocity;
double initialAngle;
/******************************************************************
 * degToRad function.
 *
 * This function takes in an angle in degrees, and converts it to
 * radians.
 *
**/
double degToRad(double angle){
    return angle * (PI/180);
    }
/******************************************************************
 * simulate function.
 *
 * Takes in the angle in radians, and the velocity. Calculates the
 * path of the projectile, and displays it in the terminal.
 * 
**/
void simulate(double angle, double velocity){
    cout << "Entering Simulation" << endl;
    double time = 0;
    double x = 1;
    double y = 1;
    double veloUp = 0;
    double veloFo = 0;
    veloUp = (velocity*sin(angle));
    veloFo = (velocity*cos(angle));
    cout << "Angle in radians: " << angle << endl;
    cout << "Initial velocity upwards (fps): " << veloUp << endl;
    cout << "Initial velocity forward (fps): " << veloFo << endl;
    while(y >= GROUND){
        x = veloFo * time;
        y = GRAVITY_FACTOR*(time*time) + (veloUp * time);
        cout << "(x, y) at time " << time << " is (" << x << ", " << y << ")" <<  endl;
        ++time;
    } //while
    cout << "Leaving Simulation" << endl;
} //simulate
/***************************************************************************
 * The main function.
 *
 * Produces output to the console in order to coach the user on what to input.
**/
int main()
{
    cout << "Execution Beginning" << endl;
    cout << "Enter the inital velocity (feet per second):" << endl;
    cin >> initialVelocity;
    if(initialVelocity > 0){
        cout << "Good. " << initialVelocity << " is a valid value for the initial velocity." << endl;
    }
    else{
        cout << "ERROR: " << initialVelocity << " is not a valid value for the initial velocity." <<endl;
        return 0;
    }
    cout << "Enter the initial angle in degrees (from the horizontal):" << endl;
    cin >> initialAngle;
    if(initialAngle >= 0 && initialAngle <= 90){
        cout << "Good. " << initialAngle << " is a valid value for the initial angle." << endl;
    }
    else{
        cout << "ERROR: " << initialAngle << " is not a valid value for the initial angle." << endl;
        return 0;
    }
    simulate(degToRad(initialAngle), initialVelocity);
    cout << "Ending Execution" << endl;
return 0;
}

正如我所说，我是C++的新手，有人能解释一下这两者是如何交互的吗？或者我应该做些什么来让它们更有效地交互。该程序编译并正确运行，但我不清楚协议以及头文件与.cpp文件的使用。此外，哪些函数和变量应该在标头的私有部分，哪些应该在公共部分？非常感谢。