If its for Johto... Go through Ice Cave to Blackthorn City and Claire will be in her gym... If for Kanto then you have to beat the first seven gym leaders then go to Cinnabar Island and Blue the sixteenth and final gym leader will be waiting to tell you that you can now challenge him in his gym in Viridian City.
The last gym is in viridian city
in mohgaony town
Blue, the last gym leader in kanto region
beat the last gym leader on soul silver in johto
Blackthorn if you are including Kanto its Viridian Pokegamer01
try the improved elite four or try to beat red on mount silver.
The fourth gym leader in Pokemon Soul Silver is in Ecruteak City.
Ghost Pokemon.
you dont
you have to use surf through passage ways
the 15th gym leader in soul sliver is blaine and he uses only fire type pokemon
you battle them in the dance center after you defeat Claire, the last gym leader in johto