It depends where you're coming from and going to, however -- in my experience -- if the airline needs these details then either they will contact you directly, or they will get Expedia to do it. This often happens a little after booking, and sometimes they will wait until nearer your departure date.
If you're concerned the first thing to try is go to the 'Manage my flight' or equivalent on the relevant airlines webpage and try and access your booking via the information Expedia gave (hopefully you have the e-ticket number or booking reference, but often just name and flight are enough). These pages generally make it clear that you need to provide further info, and you can generally do it from that page.
If that doesn't work, and you don't get an e-mail, then contact Expedia support and ask them in the first instance. They should be able to help you.