Hello! I just joined this week and my stable is quite full. What is the quickest way to earn silver so I can upgrade it? Spent about 2 days exploring, selling, and doing quests to get to 12k. 

Quests are gonna be your best friend here. Explore can work as well, but you'll need very high level pets (levels 45-50) as they will bring in the most silver.

Don't be afraid to swap out a no-star pet with a green-star pet, since the green stars mean stronger pets. Invest some money in a green star pet with decent genes on the market, and try to level it up in the arena and explore.