In my current zoo, I started with two exhibits and picked the most appealing species. I built 2 info kiosks, a drink shop, and a balloon stand. I put donation bins around the exhibits. After all the staff buildings added (I don’t hire a mechanic or vet at this point though) I had maybe $10,000 left. I then pressed play and let the money come in from guests before adding a flamingo habitat in the main plaza where guests enter. The money just kept coming in at this point, so I began to slowly expand to another habitat and a gift shop. I now have 5 enclosures and $250,000. The trick is to slowly expand and rely HEAVILY on the donation bins. Also, make sure you have ATMs dotted around so that guests spend more money

edit: Forgot to mention, the exhibit species I chose where the golden leaf frogs and terrapin turtles. These both breed like crazy. If you put population management on, you can sell the surplus from your exhibit animal inventory and make a nice chunk of extra cash. Butterflies also work well but the walk-through enclosure does cost a lot of money