Yes, and no
Yes, you can ready an action (technically called "take the Ready action", but people rarely say it like that) on your turn (which becomes your Reaction)
Per the rules of "Ready":
Sometimes you want to get the jump on a foe or wait for a particular circumstance before you act. To do so, you can take the Ready action on your turn, which lets you act using your reaction before the start of your next turn.
First, you decide what perceivable circumstance will trigger your reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it.
So you can, as your action for your turn, take the Ready action and declare that you will do something as a Reaction. This gives you great flexibility is what you can accomplish: cast a spell, fire a weapon, yell real loud, whatever. In these cases, you are giving up your Action for the trade off of doing something at a later time.
No, not everything needs to use up an Action
There are certain spells, feats, and other things that are naturally taken as a Reaction, without using your Action. An example would be the Shield spell. It has a normal casting time of "1 Reaction *". The asterisks has the qualifier:
- which you take when you are hit by an attack or targeted by the magic missile spell
So you can use it under those conditions without using up an Action to say, "I'm going to cast Shield."
Attacks of Opportunity are also Reactions.
Just remember that you only get one Reaction per round.