The world of Ground Zero presents characters with many opportunities for development. Characters will start off with levels a basic class or two, but will likely choose to pursue an advanced class once they gain some experience.
I found that the basic classes from d20 Modern lack "character", while the ones from D&D are completely inappropriate for a post-apocalyptic setting. The four classes presented here represent basic archetypes from the post-nuke genre.
(Remember that characters in many campaigns start at 2nd or 3rd level. Characters higher than 1st can select more than one class level and be multiclassed if desired.)
Also check out the Advanced Classes page. You might not be eligible for them yet, but if you're aiming for a particular advanced class you'll want to pick up the prerequisites sooner rather than later.
When characters advance, they can gain levels in more than one class. Multiclassing between two basic classes is possible without restriction in Ground Zero, but each additional basic class beyond two incurs a cumulative 20% XP penalty.
When a character chooses to multiclass, they do not receive double hit points or skill points, and they only gain enough armour and weapon proficiencies and class talents to "catch up" to the numbers granted by their new class at 1st level. All other benefits stack.
For Example: an outlander multiclassing as an enforcer would get one additional armour or weapon proficiency but no extra class talents (since both the outlander and enforcer classes start with one class talent at 1st level). A scavenger multiclassing as a thinker would pick one class talent but no additional armour or weapon proficiencies. And an enforcer who gained a level of outlander would not get any extra combat proficiencies or talents.