Emirates used to allow seat selection for free. Now things are a bit more complicated.
(1) If flying economy then the lowest fares don't automatically include seat selection prior to the online check in opening. In this case you can pay extra to select your seats.
(2) If flying on a higher economy fare or you have high enough frequent flier status then seat selection prior to the online check-in open is complementary.
(3) It's getting a bit more complicated in that it now looks like they they are also marking a few seats as extra leg room for which there is an additional charge over and above the regular seats.