class person
{
private:
    char Name[20];
    int  Age;
    int  sex;
public:
    void Register(char*name,int age,char sex);
    char *GetName();
    int  GetAge();
    char GetSex();
    void ShowMe();
};




void person::Register (char*name,int age,char sex)
{
    strcpy(Name,name);
    Age=age;
    Sex=(sex=='m'?0:1);
}
char *person::GetName ()
{return Name;}
int person::GetAge ()
{return Age;}
char person::GetSex ()
{return (Sex==0?'m':'f');}
void person::ShowMe ()
{cout<<GetName()<<'\t'<<GetAge()<<'\t'<<GetSex()<<endl;}



#include <iostream.h>
 
#include <string.h>
#include "person.h"
void main()
{
    char name[20],sex;
    int age;
    person  person1,person2;
    cout<<"Enter a person's name,ageandsex:";
    cin>>name>>age>>sex;
    person1.Register(name,age,sex);
    cout<<"person1:\t";
    person1.ShowMe ();
    person1.Register ("zhangsan",19,'m');
    cout<<"person1:\t";
    person1.ShowMe ();
    person2=person1;
    cout<<"person2:\t";
    person2.ShowMe ();
}

错误：
Sex' : undeclared identifier

'cout' : undeclared identifier


　 '<<' illegal, right operand has type 'char *'


'endl' : undeclared identifier

Sex 没有定义。你在前面写的是小写，后面是大写。
class person
{
private:
    char Name[20];
    int  Age;
    int  sex; <--- 小写
public:
    void Register(char*name,int age,char sex);
    char *GetName();
    int  GetAge();
    char GetSex();
    void ShowMe();
};




void person::Register (char*name,int age,char sex)
{
    strcpy(Name,name);
    Age=age;
    Sex=(sex=='m'?0:1);  <-- 大写。
}
char *person::GetName ()
{return Name;}
int person::GetAge ()
{return Age;}
char person::GetSex ()
{return (Sex==0?'m':'f');}
void person::ShowMe ()
{cout<<GetName()<<'\t'<<GetAge()<<'\t'<<GetSex()<<endl;}



#include <iostream.h>

#include <string.h>
#include "person.h"
void main()
{
    char name[20],sex;
    int age;
    person  person1,person2;
    cout<<"Enter a person's name,ageandsex:";
    cin>>name>>age>>sex;
    person1.Register(name,age,sex);
    cout<<"person1:\t";
    person1.ShowMe ();
    person1.Register ("zhangsan",19,'m');
    cout<<"person1:\t";
    person1.ShowMe ();
    person2=person1;
    cout<<"person2:\t";
    person2.ShowMe ();
}

真是非常感你啊．

尽量自己找错误！