First of all, I apologize if this question is already answered in the forum, but I've been looking for an answer and I didn't find it.
What I want to know is if it is possible to play an effect (like Narya) which readies a character that is not exhausted. (In the case of Narya, this is to give it +1/+1.
Also, I wonder if it is possible to discard a card to "ready" a Glorfindel, even if he is not exhausted (this could be useful to discard an elven-light)
Nope, to fulfill a "to" effect, the effect adfter the the to must be fulfillable.
This was answered definitively in a ruling about the Flame of Anor neutral event (don't have time to search for the thread at the moment). There it was ruled that you cannot play the event if you do not have an exhausted Istari character.
To be a little more precise, there are situations in which you can only satisfy some, but not necessarily all, of a card's effect where it is still allowed.