I am receiving CS5001 error: Program does not contain a static 'Main' method suitable for an entry point. how do I fix this for the following code?

namespace week11

{

class Friend

{ //

private string lastName;

private string phoneNumber;

private int month;

private int day;

private string firstName;

public string LastName

{ get { return lastName; } set { lastName = value; } }

public string PhoneNumber { get { return phoneNumber; } set { phoneNumber = value; } }

public int Month { get { return month; } set { month = value; } }

public int Day { get { return day; } set { day = value; } }

public string FirstName { get { return firstName; } set { firstName = value; } }

public override string ToString()

return string.Format("{0},{1},{2},{3},{4}", FirstName, LastName, PhoneNumber, Month, Day); } } }