I can safely say that I've started probably 100 zoos since Planet Zoo launched, probably more. The hardest thing about starting the zoo, for me at least, is the entrance. More specifically, transitioning from your entrance area to the rest of your zoo; you need to blend it in so well so it looks natural. Personally, I think the New World theme is the best for building entrances (just look at the Woodland Park Zoo's entrance and you'll see why). It just works, it flows nicely, and also looks amazing too.
As for planning the zoo, in definitely one of those people who plan the entire zoo before they start building it. Right now, I've got plans for the next couple of months, in what order I'm gonna do things, where I'm gonna put things, how I'm gonna do it. Some things are undecided, like what to do with the damn Himalayan Brown Bear and Snow Leopard. If your wondering why I'm building a habitat for the Himalayan Brown Bear, it's because I'm building a zoo with all 122 species. Now, those plans could be gone tomorrow, and new ones will be in place immediately. I'm constantly thinking about what I'm gonna do next week, next month, next year even.
Anyway, I would start your zoo with some miscellaneous animals, as I like to call them. You could make this one big area or just kinda make individual exhibits for these animals. To me, these animals don't really fit into a geographic area, or just seem like they would fit into something like this. Examples include Flamingos, Camels, Lemurs, Llamas, and the non-reptile/amphibian exhibit animals. Now, you can do whatever you want with this, but this is just how I do it. Then, break out into different areas. I prefer to break them up by geographic area, but I've recently accepted the idea of like a Great Ape area, or just a general Rainforest area, or a Reptile House, things like that.