Catch pidgey and mankey. Both level up fast. In second city you have to go school. If boss beats you, than you need to go back and call his mom. Than his mom will come to school and pull his ear.
Pidgey level up fast and it is strong. Mankey is needed for defeating Brock. If you picked charmander; you really need Mankey. After you defeat Geodude, Brock uses an Onix. Onix has fantastic def and high spd so it attacks before you and this is very weird. You can find ponyta at mankeys map and a rare grass pokemon is Budew can be find in pikachu bug map if i am not wrong. And Budew is very common in other regions. Rapidashs all stats are lower than charizard.
I think you level up faster if you dont evolve and if you use a pokemon not catched by you. So traded pokemons level up faster. Move buffet teach you old moves for 2000 so dont worry for moves. Some pokemons level up 2 times faster than others. Weak pokemons level up faster and leveling up strong pokemons is like a punishment.
Stats are base+IV+EV/2 if i am not wrong. Most important thing is you can easily raise your weak stats. HP is mostly get higher by leveling up. HP used for countering true damage. Some moves deal true dmg. You can boost your strong def and become immune to some attacks or you boost your weak def and make your pokemon balanced. Boosting high stats is a very bad tactic. Most players go full dmg and spd and we can see tanky pokemons can be immortal. Nature of pokemon directly gives -+%10 which is most effective thing and it is better if you boost your high base with this.