1. 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.
2. You need to have some healthy natural bacteria to process the wastes and chemicals in your aquarium. This will take around 4 weeks to grow naturally, but you can purchase products to speed up the process.
3. It is a good idea to keep carbon on hand in case of emergencies. Carbon can be used to soak up anything harmful that may be introduced into your aquarium. So keeping an emergency stock can be a life saver.