Go to professor oaks lab. he will give you a starter Pokemon from the johto region. They are:
Cyndaquil: fire
Chicorita: leaf
Totodile: water
after beating alder and seeing or catching all Pokemon
You only get a diploma for catching them all.
No, but you can in yellow
There really is next to nothing to do after you get all the Pokemon, but you can start a new adventure. Also, make sure to talk to the professor, usually after catching that region's Pokemon, or all the Pokemon he has something for you.
there are a lot to mention but I'll give some. in Pokemon Yellow you can have Pikachu, Charmander, Bulbasaur and Squirtle all at the same time where in Red and Blue you can only have one. Meowth is not wild in the Yellow version but can be caught in the Blue version.
Catching and Trading.
Buy the GameKiller DS Version off Amazon for $5. It gives you all the cheats you need for every pokemon game ( walking through walls, catching the proffessor etc)
Machop, Machamp, Machoke, Hitmonchan And Hitmonlee
You can complete the pokedex on Pokemon Blue version by trading with you friend who has Pokemon Red version. Or you could do what I do and get two gameboy color's and multiple Pokemon games because none of my friends play Pokemon anyomore; even though it is one of the best games of all time =).
You get the diploma from catching all Pokemon from Nationaldex and Hoenndex.
By trading and catching them i think
you get them by catching all 200 Pokemon