You put opposite gender Pokemon of the same egg group in the day care center or with a ditto. every once and a while talk to the day care man and he might give you an egg. remember to take your Pokemon out unless you want more eggs. :P hope i helped!
No, you cannot sell eggs in Pokemon HeartGold.
yes you can...you make eggs in all Pokemon games
evey Pokemon has an egg except legendarys and ditto
Yes, there is a special option in the union room
you've beaten the game gratz
You can't unless someone gives you a Darkrai for some other Pokemon you have.
Daycare center located in Goldenrod city (south)
he gives you Pokemon eggs
talk to the guy in the pink suit and he gives you headbutt
professor oak gives it to you
A guy in Maghony Town gives one to you.
Janine gives you it when you defeat her.