Okay, new player here.

I've read through the rule book and rules reference but am still unclear on how activating a card ability works. For example, a card with a reaction, interrupt, or dominance action, what is the cost to use these abilities? Do you kneel cards to use their abilities? Can someone break it down on what cost you pay to use what kind of ability? I would greatly appreciate it so I can wrap my head around this.

Activating a card ability ( X Action, Reaction, Interrupt) merely requires the declaration of its use. Costs to use the abilities will be built into the card.

For instance, Winterfell Steward's ability is:

"Marshaling Action: Kneel Winterfell Steward to reduce the cost of the next Stark character you marshal this phase by 1."

Within the activation of the ability is the cost of using it, which is kneeling the Winterfell Steward.

Ohhh, okay. I kind of figured that's what it meant, as I saw that some abilities clearly stated what action you had to take in order to use the ability (kneeling, returning card to hand, etc.). So in most cases when it isn't listed, you just declare you're using said ability. How many times can you use these sorts of abilities if they don't specify when you can (ex: at the end of the challenge phase)?

Ohhh, okay. I kind of figured that's what it meant, as I saw that some abilities clearly stated what action you had to take in order to use the ability (kneeling, returning card to hand, etc.). So in most cases when it isn't listed, you just declare you're using said ability. How many times can you use these sorts of abilities if they don't specify when you can (ex: at the end of the challenge phase)?

Addressing this point by point:

1) "Kneeling, returning to hand, etc." are not Actions, they are the costs of an Action or the effects of an Action. The Action is the use of the ability, which is dictated by the Action Window framework in the Rules Reference Guide. It is very important that you do not conflate the terms.

2) If an Action does not specify a limit, then there is none provided you can keep paying the "cost" of the action (kneeling, kneeling a house card, returning it to hand, discarding it). If a Reaction or an Interrupt does not specify a limit then you may use it once per triggering event (for instance, Aeron Damphair can only resurrect one person per Dominance Phase).

This all makes so much more sense now. Thank you so much for the help! I have seen the Action Window framework in the RRG and will study it a bit more so I don't confuse terminology and understand it more clearly.