First things first. You need to order the device and right now it might take as long as 4 weeks before your order leaves the warehouse. That gives you the time to get on the SIM card priority list. SIM cards for Ting are in "short supply" and that a visit to their early adopter thread might give you some ideas how to obtain the needed SIM card earlier.
By the time that you have the phone and SIM card in hand, a task that could take up to a month to complete, Ting will have an article up on its website detailing the steps necessary to get your Nexus 5 up and running on their network. Which is actually Sprint's network as you know. Ting says that there has been plenty of excitement around the Nexus 5 which isn't really news around these parts.
source: Ting via AndroidCentral