One possible plot could be that Ash finds Charmander much earlier than in the original series. They could form a really strong bond quickly and start training in a different way. Maybe they focus more on speed and stealth rather than the usual battling approach at first.
You can try fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'Ash starts with Charmander' on their search bars and you might find some interesting stories.
The plot could involve Ash starting his journey in a unique way. Since he has a shiny Charmander, it might attract the attention of Team Rocket more than usual. They could constantly try to steal it, creating many action - packed chase scenes. Ash would have to train his Charmander to be stronger to fend off these threats. Along the way, he could meet other trainers with rare Pokémon who are both friends and rivals, all while aiming for Pokémon League championships.
One possibility is that the Charmander was found in a special place, like an ancient ruin. It has powers that are not fully understood at first. Ash, being the determined trainer he is, decides to figure out how to use these powers to their full potential. As they journey together, they face challenges not only from other trainers but also from mysterious forces in the Pokemon world that are interested in the Charmander's power. This fanfiction could be full of adventures, growth, and the development of a strong friendship between Ash and his Charmander.
It would change his journey a great deal. Firstly, having a shiny Charmander would make him stand out more. Other trainers would be more likely to challenge him, either to prove themselves against someone with a rare Pokémon or to try and get his Charmander. Also, trainers with more experience might offer to help Ash train his shiny Charmander, which could lead to new training techniques. In Pokémon contests, his shiny Charmander would be a show - stopper, giving him an edge in those aspects of his journey as well.
Ash would likely be more confident from the start. Since he has a powerful Charmander, he might be more eager to take on stronger opponents. He would also have to learn how to handle a Pokemon with such great power, which could make him more responsible.
In this fanfiction scenario, Ash and Charmander could have an adventure in a volcanic area. Since Charmander is a fire - type, it would feel right at home there. They might discover a new type of Pokemon that is only found in volcanic regions. Also, Ash could participate in a local fire - type Pokemon tournament in this area. With Charmander's growth and training, they could potentially win the tournament and gain some rare items or Pokemon eggs as rewards.
His relationship with Pikachu might become more competitive at first. Since Ash already has a strong starter in Charmander, Pikachu might feel the need to prove itself more. But over time, they could learn to work together better, with Pikachu using its electric attacks to complement Charmander's fire attacks in battles.
In this fanfiction, it could be that Ash has a really good strategy. He might first make Gyarados use up a lot of its energy by making it attack Charmander's illusions or decoys. Then, when Gyarados is tired, Charmander can step in with its full power. Charmander could also use the environment to its advantage. Maybe there are some flammable objects around that can be set on fire by Charmander, creating a more powerful and widespread fire attack that Gyarados can't avoid.
A key element is the bond between Ash and Charmander. It's usually shown as a strong and unbreakable connection. Another is their growth. Ash grows as a trainer and Charmander as a Pokémon. For example, Charmander evolving into Charmeleon and then Charizard is often a big part of the story.