Choose a top-notch primer like Zinsser 123, C2-One and apply a good/full coat.
* Aim for ~ 250-300s.f./gal for this step. Those walls will be VERY thirsty.
* PVA-class primers are OK, BUT....get plenty of it on the wall!! I'd do TWO coats of this stuff.
* If the price is less than $15/gal., walk away & look for better stuff.

OPTIONAL:
* For really sexy smooth walls, let primer harden for a day, and lightly screen/pole-sand them with a 180-grit screen.
* This knocks off little fuzzy nibs when a water-based product hits the sheetrock paper.
* Wall will have to be vacuumed/rinsed again here b4 final paint!
* End result will be quite good looking though!

If you're planning to shoot texture of some type....prime, shoot texture, and prime again. Then paint.

Faron

Faron, will the optional sexy-smooth step work if we prime walls that are already painted? (House is three years old and has knock-down on the walls.)
Thanks!

If ya have textue on the wall now, you'd be sanding a while!!
If you want smooooth, you'll (ideally) prime the existing wall, hire someone good at skim-coating/plastering, pole-sand the walls, vacuum & remove all dust....then prime again.
>>> At long last, you're ready for 2 coats of final paint!

Yes...quite a process!

Sanding AFTER priming can be an advantage, in that some primers will "fuzz" the paper more than others.
If it were MY walls:
* I'd apply good full coat of an Enamel-Undercoater primer.
* Let dry ~ 6hrs. to be easily sandable, using ~ 220-grit sanding-screen.
* Fair amount of dust here, so make sure you have a Fine-dusts insert-bag for your Shop-Vac!!
* Lastly, lightly sponge-rinse the walls after a thorough vacuum.

>>> NOW you're at the "sexy-smooth" wall stage...ready for 2 coats of finish-paint!!

Faron