For a 1-action spell, do I need to take a turn to ready the spell before I can cast it, or can I cast it immediately?

I am still very new to all of D&D 5e’s rules.

Let’s say I have a wizard that knows fireball; he has the spell prepared and has an available spell slot. Basically, assume ideal for everything.

Do I have to take a turn to “ready” the spell, then cast it the next turn? Or can I cast it right then and there?