In the world of Pokemon money is important thing. You can buy items, pokemons, pokemon caretakers, car. You can also buy houses to yourself or for your guild all over the region or maybe even all over the world. You can hire mercenaries to make your job, basically you can do almost anything with the money.
Good Way
The question in the air now is "How to gain money?". There are a lot of ways to gain money. The most simple one is probably to fight with fellow pokemon trainers. When you have won then the opponent must give 25% of his money to you. If you win a Gym Battle you gain 10000$. Breeders can earn money by healing pokemon trainer's pokemon and then get some of their money. Pokemon trainers, coordinators, breeders can also get money by winning contests. If you win against Elite Four then higher ups give you the money. You might also get lucky while traveling and find a purse or something like that on the ground. You can also get money by working with your guild members as they can donate some for you for a little while or you win some kind of group event with high pets. Basically there are a lot of ways to gain money, you just have to wind intelligent ways to get it.
Evil Way
The evil way might be easier but remember, you might be throw in jail for it. The most basic way for evil trainer to gain money is to beat up the opponent trainer and then take all of his money. You can also do NPC stealing missions and get money or go into evil group and start doing ... evil things. There are a lot more Evil Ways making money than there are Good ways of making money, you just have to put your brain working.
Everyone will start with 10,000$