Blue Monday - majority of the time, people buy their snorkel, mask and fins first, the rest comes at a later date. if you haven't got those three things as yet, I would highly recommend getting them.

Now with the rest of the equipment, it's entirely up to you, from what others have also suggested on here in other posts is to get used to the equipment first, use the rental stuff and try out different types/makes/etc and see what you like about it, what you don't like about it.

HTH