I don't think you can.
no, in Pearl as well as Diamond and Platinum, you can only be in the Sinnoh region
johto and kanto
You can't get there because you can only go to johto and kanto.
Mainly Johto, but after you beat the Pokemon League you can go to the Kanto.
basically its every Pokemon from the kanto region to the johto to hoen region
== ==
All the Pokemon that didn't exist in that particular region. Example in firered and leafgreen you can get johto and hoenn Pokemon like lugia or rayquaza. In diamond and pearl you can get stuff from kanto and johto and hoenn.
Get all gym badges of Johto and defeat the Pokemon league. Then you have access to the Kanto Region.
you trade with others who has the kanto, johto, or other hoen regions
No. Johto is not an available region in Pokemon FireRed. You can only explore the region of Kanto and the nine islands surrounding it. Currently, only Pokemon Gold, Silver and Crystal are games which explore Johto.
you start out in the johto region if you mean kanto you have to beat all the gym leaders and the elite 4
493 Pokemon total Kanto:151 Pokemon Johto: 100 Pokemon Hoen: 135 Pokemon Sinnoh:107 Pokemon