Once you reach level 20 and, this is revealed by datamined info, get a Vex item drop, you recieve an invite to support factions. You then go to whichever faction you want to support first, and buy their class item for (i think) 250 glimmer. When you earn Crucible and Vangaurd XP and Tickets while wearing the class item they convert to that Faction's XP and Tickets. There is also non-faction "psuedo-factions" that operate like this, the Reef Gaurd is one of them. [spoiler]there is also patrol missions to boost faction levels (I would assume early since you can get Old Russia Faction missions at level 4)[/spoiler]