In Mahogany Town buy a RageCandy Bar from the man guarding the exit. Now go Goldenrod City and ruin Team Rocket's Plans (check the link for helphttp://wiki.answers.com/Q/How_do_tou_get_in_to_the_radio_station_in_pokemon_heart_gold). When you defeat them, the man that you bought the candy bar from will let you through, now you should be able to get to Blackthorn City to battle them.
You don't have to buy anything from that man, but if you want his candy bars stock up- he won't be selling them after you beat Team Rocket. If you talk to him he will say he's out of stock. They're not that useful anyways.
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dragon denyou can find dratini by surfing in dragon's den in blackthorn city. (the city where you get the eighth gym badge)
you capture the ledgon dariy Pokemon
go to the next gym leader.
Go to the forth gym!
lv 30