First you need to get 5 badges. When you beat the 5th gym you can talk to your mother to get the amulet coin which makes getting money much easier. But by this time you would probably have enough money. It's not a good idea to try and get lots of money early game because it's a lot harder.
First catch 4 zigzagoons on route 101 because they have the pickup ability which makes them have a chance to pickup an item every time a battle ends. Then teach a Pokemon the thief tm which you should have obtained from the oceanic museum. Make sure the Pokemon with thief isn't holding an item. Give a different Pokemon the amulet coin.
Now go to route 104 and rematch rich boy Winston. If he doesn't want to battle run around a bit and try again until he does. Use thief to steal a nugget from his Pokemon, then switch into the Pokemon with the amulet coin and beat him. He will give lots of money and you can sell the nugget for 5000 pokedollars. Now you can also check if the zigzagoons are holding anything so that you can also take and sell those items.