First you need to get to one island when you get there go towards mt ember and stay in the grass you will eventually find ponyta.
you can't get it from a friend that has firered or leafgreen
in the video game section
No you can only get it at the beginning of the game.
Ponyta is not in the game.
you can't get a shellder in leafgreen, you can only get it in firered. you can ask a friend that has that game to catch you a shellder and trade it to you.
you have to buy a game shark
search it on youtube
in the safari zone or you can buy it at the game corner
you can't only if u have a game shark
you can't CATCH squirtle in leaf green. but it can be chosen as starter Pokemon at the beginning of the game.
you need to start out with bulbasaur at the start of the game, beat the game, get the rainbow pass, encounter then track entei then try to catch it.
As far as I know you have to evolve it from Ponyta (Lv. 40) catch ponyta north from the town with the place where they look after your Pokemon for you transfer it from a GBA game (Like Saphire or Fire Red ect.) or trade. You can catch one when you go to mt. silver from the indigo plateau