The first conditional describes a particular situation, whereas the zero conditional describes what happens in general.
For example (zero conditional): if you sit in the sun, you get burned.
(here I'm talking about every time a person sits in the sun - the burning is a natural consequence of the sitting)
But (first conditional): if you sit in the sun, you'll get burned.
(here I'm talking about what will happen today, another day might be different)
The first conditional describes things that I think are likely to happen in the future, whereas the second conditional talks about things that I don't think will really happen.
It's subjective; it depends on my point of view.
For example (first conditional): If she studies harder, she'll pass the exam.
(I think it's possible she will study harder and so she'll pass)
But (second conditional): If she studied harder, she would pass the exam.
(I think that she won't study harder, or it's very unlikely, and so she won't pass).