To be a herpetologist all you need is an interest. Amateur herpetology is something of which all of us can claim to be a part.
If you're talking about being paid for it, well, like everyone has said so far, there are no jobs for "herpetologists". The greatest historical "herpetologists" were most often also ichthyologists and ornithologists, and herpetology developed as a recognised science by the works of these men in their free time or in retirement. It was always more of a hobby than a respected discipline.
My avenue, as I've always aspired to being a herpetologist, is doing my Vet degree and specialising (in my own free time, because no one at Uni can teach me) in Herpetological Medicine and Surgery.
Buy heaps of books, read them. Speak to heaps of people. Go to heaps of places. Study lots of animals.
Then publish something, and everyone will think you're a herpetologist ^^
There are only a handful of universities in the world that offer any kind of herpetology degree. I've heard of one in Texas and one in Queensland (although this one could be a myth).
As for it being a growing or shrinking profession.. well, it's not really a profession at all. But the interest and the research is increasing exponentially. I'm actually almost worried people will pinch all the great research ideas I've got for the next 20 years
