The zero conditional is used to say say that "If "x" happens, "Y" will also happen. For example, if water is heated to 100, it will boil, or if I eat too much, I will get fat. It is an absolute- if one thing happens, the other thing must follow.
The second conditional is more tricky, but is usually only used in formal speak and isn't very common. It has two uses. The first is to talk about things in the future that are probably not going to be true. For example "If I were king, I would live in a castle". The second use is when you are talking about something in the present that is impossible because it isn't true. An example would be something like "If I had enough money, I would buy the T-shirt now".
To be honest, you don't see them get used very much as they are involved in quite formal language. Hopes this helps a little.