Firstly, note down where and when you found it, this is especially useful for protected species such as birds of prey.
An animal is only good for taxidermy if it is fresh and in good condition, so as soon as you have it, it needs to be frozen. Though a long dead animal or an animal that’s been flattened won’t be any good for mounting.
Wrap the specimen in a plastic bag (removing as much air as possible) or into a clean tupperware for very small animals, this reduces the chances of dehydration and freezer burn down the line. Mark the container with the information you collected and put in the freezer as soon as possible.
See my How To guide for more information!
If properly stored the specimen can last for many years frozen. (Side note: UK Taxidermist Jazmine Miles Long once mounted a Cheetah skin that had been frozen for a whopping 37 years!)