What do you mean, exactly?

A number #\alpha# is said to be rational if there exist two integer numbers #n# and #m# such that #\alpha=\frac{m}{n}#. In particular, all integer numbers are rational numbers (which is what we mean when we say that #\mathbb{Z}\subset \mathbb{Q}#), because you can choose #m=\alpha# and #n=1#. And 0 is no different from all other integers: you can pick #m=0#, and for any #n \ne 0# you have that #\frac{m}{n}=\frac{0}{n}=0#, and so 0 is a rational number. If you can explain what you exactly meant with "what kind of rational", I'll be glad to answer:)