Water or Blood
Not every Ritualist has access to all rituals, and without extensive work, research, and resources one cannot design their own. Many organizations zealously guard their rituals and only teach them to initiates of their order. The Librarians, Sovengers, and even the Church all have their own tomes and traditions, though these all share some common rituals. Talk to your GM before learning any one ritual.
Characters have a number of rituals equal to their skill ranks in Knowledge: Arcane,
If they have feats in other magical trees they may take Knowledge: Blood Magic /Necromancy /Binding /Summoning /Geomancy /Divination, or possibly stranger. This only counts your ranks in the skill and not other bonuses, with one exception: the Feat Skill Focus grants three additional rituals.
Spells labeled with Blood Magic or Necromancy are heretical and require spending at least one Blood Token or Spirit Token respectively before any roll is made.
Most Rituals require a fetish/focus, and captured fetishes/foci can often be used to learn the spell they have channeled. Many ritual focus/fetish have a common form that is taught for channeling the spell and these examples are listed with the spell entries.
Minor (DC up to 15), Major (DC 16-25), Epic (DC 26 and up)