These values will be set regardless of what your relations with those factions was before entering the service of the king. Once you have sworn your oath to a king his enemies will become yours your relation with the faction you just joined will be set to 12, while your relations with those at war with your new faction will be set to -40. You can also enter the service of a king by becoming a Mercenary Captain and earning enough Renown to become a fully-fledged vassal. Once you have sworn an oath to a king you will no longer get offers from other rulers. Once you have built up this amount of Renown you will start getting offers from kings interested in having you as a vassal refusing an offer lowers that king's reputation of you, and if it falls below 0 he will no longer offer vassalage.
In order to join a faction you will need to build up at least 150 Renown raising your relations with the king will allow you to join with a lower amount of Renown(5 less renown needed for joining per relation point).
