You cant.
If you complete the Hoenn Pokedex you can get a Joto starter, or you can them by trading Bayleaf from Pokemon Colloseum (after it is pruified) from Gamecube to leafgreen or firered, then breed it, then trade the Chikorita to Emerald, the to Ruby! It's very complicated, but it's the only way without cheating!
You have to trade with friends to get them, or trading with someone else (it can be by using GTS too) or by having another compatible Pokemon game (another LeafGreen, or a FireRed for instance), where you can restart the game and trade the starters.
Upstairs in the Pokemon Center?
you cannot catch celebi in Pokemon leafgreen. you can only get it through cheat codes or trading.
Only by trading.
As always, it involves trading with another version of firered or leaf green, or older games
unless you use a gameshark/codebreaker, you cant, sorry.
Only by trading it from Leafgreen/Firered.
Only by trading from LeafGreen and/or FireRed
It is not possible to get all three starters in Pokemon LeafGreen without trading. This was an intended mechanic to encourage interaction between players.
Machoke, Kadabra, Graveler, Haunter... Loads...
Teddiursa is not obtainable in Leafgreen and the only way to obtain is from trading Teddiursa from either Pokemon Emerald or Pokemon XD Gale of Darkness.