all you have to do is beat the Pokemon league 1 time its simple.
You need to get 100 pokemon then go and talk to professor Birch and he will upgrade your pokedex.
go to Google and search Pokemon sapphire pokedex and it will give you a list of all the ones you can catch, you can also search for a national pokedex and look for treeko and that is the beginning of the sapphire/ruby/emerald pokedex(heres a link of the best one ive found sofar ... http://Lloyd.truepath.com/Jonathan/PKMNRS_Pokemon.htm ... ) go to Google and search Pokemon sapphire pokedex and it will give you a list of all the ones you can catch, you can also search for a national pokedex and look for treeko and that is the beginning of the sapphire/ruby/emerald pokedex
I think you need to see them all.
Unfortunantely, it's impossible to catch ANY Pokemon from the National PokeDex in Ruby and Sapphire. You have to trade with FireRed, LeafGreen, Emerald, Colosseum, or XD to get Pokemon from the National PokeDex. If you trade with the first three games, you'll actually get the National PokeDex in Ruby and Sapphire.
trade it from ruby sapphire or emerald after getting the national pokedex.
you cant find it, you have to see all the Pokemon sapphire.
you can't unless you get the national pokedex and trade for one in sapphire or ruby.
You have to use the Pal Park option which is only available after obtaining the National Pokedex. Even then it isn't trading, you are transferring Pokemon from Sapphire to SoulSilver.
You get the national pokedex and a jhoto starter.
You can do many things after getting the National Pokedex. You can find the Ruby and the Sapphire, give them to Celio in One Island's PKMN Center and trade with Ruby/Sapphire/XD/Colosseum. You can visit the rest of the Sevii Islands, find pokemon not native to Kanto, and much more.
You do need the national pokedex for the Pokemon modifier. the Pokemon modifier uses the national pokedex numbes. you can not use the sinno pokedex.
You can link and trade Pokemon with Sapphire and Emerald Versions. If you want a national pokedex, trade with leaf green version