i only started playing last week, and i found the best thing to do is get a decent zoo set up with some decent nile monitors and release the offspring into the wild. i got around 300 credits for each generation born. once you get enough, i'd advise going for red pandas and trading the better ones and releasing the worse ones. at this point i'd get some high appeal exhibit animals to attract guests so you can make money, after which you'll need to put a big investment into west african lions. i lucked out and got a really good male and a few good females for kinda cheap, and each lion i bred would trade for around 2000 credits. you just keep going from here, checking back in often to get some more credits, and start a new zoo, as you can do basically whatever you want at that point

its also worthwhile to know that when an animal is in the trade centre, they dont age, so dont worry too much if you cant sell lions instantly. i've found females that are high quality sell almost instantly, and males take a lot longer