You catch a ditto to breed your starter with it.
the champion gives it to you before you enter twist mountain after beating cheren
Before you beat the league, the only things you can transfer from GenIV games (inc. HG/SS) are the 13th movie Celebi and shiny beasts, using the Relocator. Only after beating the league do you get access to the PokeTransfer lab.
no you get to chose 1 in the game and chen and (the other girl) you will see they evole the starters and you can get them in the globel trade
You can transfer Pokemon, but you can't trade. Since HeartGold was released before Black, HeartGold doesn't know Black exists. However, Black knows HeartGold exists. You need to be on Route 16 in Black version, which requires beating the Elite Four.
after beating alder and seeing or catching all Pokemon
you can transfer Pokemon from Pokemon platinum to black and white but you can't trade from black to platinum
After beating all the Gym Leaders, complete your pokedex.
you get out the charge stone after beating N
Humilau city after beating Marlon
After beating the Nacrene City gym
you can find it in pinwheel forest after beating the elite 4 (.)(.)
you can't. there could be an event in the future or something