You should go heal your Pokemon. Then go behind Clair's gym and use surf and go into the cave which is called the dragons den. After doing that you need to get into the back part of the house in the water by using whirlpool on the left or right side of the house. After that you need to talk to the elder he will make you do a test the answers are:
Q1:friend
Q2:strategy
Q3: strong
Q4: love
Q5: Both
IF YOU ACE THE TEST YOU ALSO GET A DRATINI WITH EXTREME SPEED AS AN ATTACK after going out of the dragons den and going back in there with a free inventory spot.
I think you have to find "The Dragon's Fang" inside The Dragon's Den. When you find it she will come to you and give you the badge and a TM.
Go to the forth gym!
It is attainable in poke mart after the 3 gym badge (i think)
In one of the gyms. I don't know witch one.
There is no best Pokemon on Pokemon HeartGold.
there is no Pokemon Mansion in Heartgold.
Go to the forth gym!
You will have to defeat Bugsy in Azalea Town. He has bug-type pokemon.
it is bugsy the bug type leader
In Mahogany town right from Etrack city
blackthorn city
It is your rival from Pokemon fire red and leaf green versions.
you have to get the cianwood gym badge and the olivine gym badge
in Goldenrod city
you get it in ilex forest after you get your 2 badge! hope this helps
go to the sprout tower in the town where you get the first badge at night.
It is in Ecrutuk city the leader is morty who uses ghost type Pokemon
It is attainable in poke mart after the 3 gym badge (i think)