I presume you booked the UA codeshare on united.com — if that's the case to select seats what you can do is go to the ANA webpage and pull up your booking by putting in your UA ticket number (starting with 016). Note that if you have a middle name you'll probably need to concatenate it with your first name for you to find the reservation.

Once you've pulled up the reservation on the ANA site you should be able to view the NH (not UA) PNR. Call ANA and give them that PNR and they can assign seats for you.

NRT-SFO-NRT is usually the 77W with 3-4-3 in Y (rows 24 to 42), seatguru v4. However IME the seatguru seatmaps for NH aren't very accurate so I wouldn't put so much faith in their comments and the like. Also note that for NH you can reserve the exit rows for free if you book early enough, so that might be an option if you really need the legroom / don't mind the fact that they're very narrow (especially in the 3-4-3 config).