I am trying to find out about luggage and about getting it checked in

Your question is not clear could you ki ndly ask it again and ask what exactly you need

I am flying United from YUL to NRT and transir in IAD. Do I have to check out my luggages and check in again at I AD? Thanks for helping.

In your itinerary look for airline name to checkin your baggage

Check your bags before you go through security. Check with your airline for baggage fees