These two routes contains a massive amount of trainers that you will have to battle in order to reach Bill's House. Both of these routes contains Grass piles and watery parts that provide a big variety of Pokémon.
Route 24 starts at the bridge at the north part of Cerulean City. On this bridge you will have to fight five trainers in a row and then a Team Rocket Grunt will ask you to join Team Rocket. If you go west after the bridge, you will have another battle and the grass piles and water that contains this route's Pokémon.
Another Pokémon that is exclusive only to Pokémon FireRed. Combining Grass and Poison type makes him a very useful Pokémon in your team. Most of his attacks include absorbing attacks which are very useful for long fights without a place to heal, and powders that are helpful when you try to catch other Pokémon. After you defeat the Elite 4, you can evolve Oddish into two different Pokémon, Vileplume and Bellossom.
A Pokémon LeafGreen exclusive. It uses mostly powered attacks which are great when you try to catch Pokémons. It is a Grass and Poison type Pokémon, which gives it extra resistances and only a few more weaknesses. Bellsprout is great if your Starter Pokémon wasn't Bulbasaur.
#63 Abra
The first Psychic-type Pokémon in these games. It is very sneaky, so when you try to catch him, you can either throw a Poké Ball or try to put it to Sleep or Paralyze it. If not, it will Teleport away. To evolve Abra to its final form, Alakazam, you must Trade it with someone.
Route 25 starts a bit east to the end of the bridge on route 24. It's got even more trainers and at the end of it you can find Bill's house. When you get to Bill's house, you will see a Clefairy. Talk to him and he will go inside the machine. Then, go the PC and press . He will come out from the other side as Bill. Talk to him and receive the S.S Ticket.