After you figure out if you have saved up money and think it would be useful to you in what ever way e.g. you current machine is getting slow, you like new shiny stuff, you might be a photographer and what more colour gamut here are somethings to consider.
Perhaps the only reason to get the new Macbook Pro is for the improvement in the screen brightness and colour gamut.
USB-C to SD Card adapter
The first thing you will need to buy extra with the new Macbook is a USB-C ( aka USB type C) adapter, there are few different options from ones that include ethernet ports, USB A etc. If you are a photographer you may also need to get the SD card adapter.
Ethernet and USB-A to USB C adapter
When should you buy it? It is realistically not available for another month or more. So that means not until perhaps 31st of November 2016. So that should give you time to prepare.
How much internal storage do you want? given the extreme pricing of internal storage for many markets it may be worth consider maxing out at what ever your current MacBook Pro is, so that might mean between 256GB and 512GB for most people.