Check your bags straight through to your final destination. Don't let your airline tell you that your layover is too long for this. Narita baggage handlers are used to it and will keep your bags for at least 24 hours.
Pack your carry-on with just what you will need during your layover. Don't forget a change of clothes if you're staying overnight. Remember, Japan's weather in January is somewhat different from that of Thailand....(It ranges from below freezing in Winter to 40C in Summer.)
Ask your airline if your ticket permits you to leave the transit area. Some tickets do not show the intermediate stop, meaning you may be stuck in the airport.
If you are staying overnight, ask if your airline provides a hotel room. Some do provide both food and accommodation.
Make sure you get boarding passes that cover your entire journey; both to Narita and onward to your next destination.
