For more information on double-faced cards, see the Shadows over Innistrad mechanics article (http://magic.wizards.com/en/articles/archive/feature/shadows-over-innistrad-mechanics).
Your starting life total is the life total you began the game with. For most two-player formats, this is 20. For Two-Headed Giant, it’s the life total your team started with, usually 30. In Commander games, the starting life total is 40.