I'm not one of those people that can plan things out on paper well, but it may help. Try drawing an outline on paper, deciding what theme goes where, where the guest areas should go, etc. It can help if you're not so organized.
Me; I'm more of a spontaneous person. I'm not good at assigning places for particular things before hand. I just build and go from one square to the next, randomly deciding what goes where.
The trees, plants, rocks, and scenery objects do wonders for the game. Don't leave many spots bare..fill it with a patch of flowers and a pretty tree. A measley square of grass can be transformed into a beautiful square with a neat tree and pretty flowers.
Don't be afraid to get creative! Remember: you always have the bulldozer option if you mess up. Don't be afraid to make curvacious paths that wind through foliage, hills, and rocks. Viewing areas are by far one of my favorite areas to build. Theme it to the exhibit it offers a look into. Add viewing areas, benches, donation boxes, and don't forget...TREES!
Exhibits are a great opportunity to unleash your creativity. Depending on the animal you have, just think of it's original habitat, and even some real life exhibits you have seen. I find building a two story watefall dropping down in sections is a wonderful visual! Remember to add hills to your exhibits as it will make them look more natural. Leave an area for guest's eyes and fill it with the enrichment objects and food the animals prefer.
Hope my two cents helped somewhat