You will have to defeat Bugsy in Azalea Town. He has bug-type pokemon.
it is bugsy the bug type leader
If you mean the Wifi Plaza, then it opens after you get your first or second badge I think.
Go to the forth gym!
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)