Do all animals mate?
Higher animals, reptiles, and mammals mate because they have two genders. Fish, however, almost always fertilize their eggs after they have been laid.
Mating is an adaptation for the perpetuation of species.
Sexual division increases the genetic variations. Plants also have sexual reproduction, but do not technically "mate".
Some animals reproduce asexually so they can produce offspring without mating. Animals such as the hydra can reproduce asexually by budding, producing a clone of itself.
Not all animals go into heat, otherwise known as estrous. This is only something you'll find in mammals. Other animals might have seasons when they are able to mate and other seasons where they are not able to mate, as part of their life cycles, but the term "heat" is reserved for the mammalilan estrous cycle, which is based on hormonal controls.
Put a male and a female (same animal) in together and let them do the rest. Animals wont mate unless all their needs are satisfyed. Sometimes, they will mate and nothing will happen, it takes time. Some animals can cross breed to create new animals. I think this mainly happens with User created animals however.