737-3/4/500: 3 levels of your choice, 'traditionally' Fl200/150/100 but often lower ones not needed where LNAV/VNAV not needed with radar vectoring eg London TMA below FL160.
Normally (in my experience) obtained from forecast winds printed on PLOG. FMC 'knows' cruise wind so that is not needed for descent, and if no winds entered will factor (2/3 of cruise applied at FL200?? I seem to remember).