How do I obtain new jobs?

For Marche, I have the required abilities to become a ninja, but I can't find the job in my job changing ring.

You need to have two Thief abilities mastered to become a Ninja. Have you done that?

http://www.eyesonff.com/ffta/jcprogression.php

I got "Steal: Weapon" and "Counter" mastered, but the ninja job class didn't show up.

Counter isn't the right type of ability.

Counter is an R-Ability. You need two A-Abilities. So learn another type of Steal.

Thanks, now I know why my templar won't become a dragoon.

To become a Ninja you need to A abilities, I suggest Steal: Gil and Steal: weapon, Also, Before you make him a ninja, make sure he has counter and combat combo.

Do that with another soilder and then keep them close together for the combos.

For Pladains, you need 2 Soilder Abilities.

And for the Bangaa's, 2 White Monk Abilities gives you Bishop and something else, and 2 Warrior abilities gives you Templar, Dragoon and Defender [I think]

"Progression - Job Class - Final Fantasy Tactics Advance - Eyes on Final Fantasy"

Is Wrong, on one part, you need Two archer abilities to become a hunter, not one as the site suggests

Go tell the site staffers in the Site Volunteers forum that the page is wrong :) That'll do more good then posting in threads from january xD