[RPG] How does uses of Channel Energy work for a multiclass Cleric/Paladin

Yes as it states all creatures of one type are effected.

To target selectively a cleric should select the Selective Channeling feat which allows them to exclude targets up to their charisma modifier from the effect.

Clerics can also modify who they target using the Alignment channel feat to target outsiders instead.

With Channel Energy (read the linked rules), it is a standard action. You have to "present" your holy symbol - which doesn't quite qualify as requiring verbal, somatic, or material components so it works even in Silence or if bound unless your GM rules it doesn't - and the channeled energy goes out from you in a 30' radius. There is no roll to make it work per se, and it's even better than a spell in that it can't be disrupted, doesn't provoke attacks of opportunity, and doesn't need Concentration checks to do in combat.

If you have chosen to use it to heal, it heals all (even enemies, unless you have Selective Channeling) in the radius. If you have chosen to use it to harm undead (if good) or creatures (if evil) same deal, it hurts all in the radius, at that point their make their save for half damage. The DC of this save is equal to 10 + 1/2 the cleric's level + the cleric's Charisma modifier. That's clearly spelled out in the rules linked above, you should go carefully read them.