1. When you purchase your aquarium and the equipment for your setup, do not purchase the fish at the same time. You are going to need plenty of time to cycle the water in the aquarium to make it hospitable for the fish in your aquarium.
2. You should always purchase the correct lighting setup for your aquarium. Doing this will make the habitat much better for the fish and plants in your aquarium.
3. It is generally a good practice to purchase the largest aquarium that you can actually afford. A larger aquarium will be less susceptible to quick changes in the chemistry, and will offer much more stability.