It sounds like you're not a member of the Executive Club. Advice -
join it this weekend. On 1 July the rules will change so that you will have to fly one full(er) fare economy (or higher) sector just to be able to join.
As an Executive Club member, you will be able to check in online 24 hours ahead instead of 12.
To see the seating configuration for your flight before check in opens, go to
www.seatmap.com and put in all your flight details. (Date, airline, flight number, class - Y will do for the main economy cabin - origin and destination
airport (not city) codes. The system won't work unless you put in all the details.) Between this and what's on the airline website, this should point you in the direction of the seats you want.