Doing quests, playing the Auction House, providing crafting services (with your professions) are all ways to make gold. Now weather or not you can make tons of gold is up to you. You just have to...
You have to train on the monsters according to your level. i will include the website (in the links below) where everything that you need to know and want to know is included.
To digivolve in Digimon world 4, you have to "unlock" a new digimon. There are several "Secret" missions that require you beat the game, have specific perficiancies, ect.