Once you can do West Dickens quests (when you see a W on the map... start his quest line.
Once you get far enough ( not too far in) you open a quest in Armadilo. The quest is a question mark and is behind the movie theatre (old church) in town.
Start the quest and go to Thieves Landing, be sure to have $200 on you. Play Dice to win the deed for the quest.... usually you can only bet $20, but if the quest is active you can bid $200. The dice game is easy, I have not lost yet, and I made 10 grand for me, and 10 grand on my sons game for him.
Do not leave the table and save until you are happy with your winnings. You win $600 per win.
Other good money makers.... Kill cougars and sell the items (claws, fangs, pelt, meat) it ampount to almost $50 per cougar. Kill bucks, not as good as cougars, but still good. Kill hawks, $5 per feather.
You can buy weapons, maps of areas for fast travel, buildings/rooms, rides via stagecoach, items like apples for horses, bait for hunting. You can also buy horses, but I would not, its expensive, and you can just go catch the fastest horses.
Do Bonnie McFarlanes quest line until you get a laso. Then catch a good horse (pure black, pure white, or the one she lets you keep... it is gold) these are 3 star horses. They sell for $750 to $1500 depending on your fame. Be sure to hitch your horse after you catch it to make the horse your permanent ride.