Buy or grow green peppers then cook them in a frying pan. Sell them for about 300G. Buy three horses and train them at 2 stars, 5 stars, and 10 stars. Use them all in a horse race and then sell them. Pick up coral at the beach. Take them to the blacksmith and order a brooch. In two days, get it, sell it, and make about 600G profit. Have four chickens and make four deviled eggs a day to earn about 1,000G. During winter, go to the lake cave. Go to floor 50. Fill up your pockets with diamonds, then go home. Sell them, get more stamina, and go back. Repeat this about every day and go to bed at 6:00 p.m. With the large rucksack, you should make about 10,000G a day.
Winter time is probably the most profitable season with the access of the Lake Cave. Each day is precious, especially in year one when your wealth isn't at it's best yet. Here are Lake Cave "Specialist" Sublevels. These are levels that only contain a specific gem.
This is all I could find so far~ I think level 30 is Sapphire(250g) but I don't fully remember. :D Hope this helps a ton b/c I couldn't find any info on the best levels before my findings. Good Luck on getting to level 50, I got there and managed to get 7,500g out of that one level(I had the 3rd bag expansion) .
You wont get a free horse like some other Harvest Moon games. You have to buy the horse but before you do you need a barn which you can buy at Woodys Shop. He will start as a foal. You need 2 hearts for you to be able to ride him. When you get 12 Stars you can seel for 50,000 dollars.
**Easy way to get 12 Stars is after he has two and you can ride him. Stay in the barn and run into a wall. You wont go anywhere. Also every 12 hours you ride him(doesnt have to be in a straight row) you get 1 star.