To get Totodile in Leaf Green / Fire Red, whichever game is your favorite version, you have to complete the Kanto Pokedex', which is only possible by having both Fire Red / Leaf Green. After you get all the Pokemon of the Kanto Pokedex, Professor Oak will let you choose only one of the three Johto starters, which are: Totodile ; Water, Cyndaquil; Fire, and Chikorita; Grass. Remember, you can only choose ONE. I don't know whether your rival comes and chooses one as well. If you want ALL of the three starters, and if Fire Red / Leaf Green is your main Pokemon game, on Fire Red / Leaf Green complete the Kanto Pokedex' and trade the Johto Pokemon over to the game that is your "main" Pokemon game. Do this again, by getting a different starter that you didn't get yet. Note: This means erasing the game and starting all over. Other than this, the only other choice in hacking or using GameShark and Action Replay; which I don't recommend. Hope this helped! Happy hunting!
You first have to catch all the pokemon in your kanto dex. As a reward professor oak will give you one out of the three johto starters ie Totodile, Chikorita or Cyndaquil