NB : You can select your First X1 and 3 Subs without using any of your subs...And once you have saved the First X1 you will have 30 Substitutions to play with through the First Segment which will end on 2nd of January 2016 (Gameweek 20)
Second Segment will begin on 12th of January (Gameweek 21) and you will get 30 more substitutions to play with during this segment.
For all your doubts regarding the Points System and how the team combination should be please refer the below link