Level 0 Characters
Source GM Core pg. 84
Before they were heroes, every PC came from somewhere, whether they poured coffee in a dusty, sun-scoured village like Chk Chk or performed in a busy spaceport like Dae. Sometimes, it can be a lot of fun to play a prequel game set years before the PCs' first adventure as heroes, or you might have an idea for a low-powered adventure. The rules below provide ways to build and use level 0 PCs.
Building Characters
Source GM Core pg. 84
Building a level 0 character is similar to building a 1st-level character, but you stop after choosing your ancestry and background. A level 0 character still gets the four free attribute boosts from Step 6 of the normal character creation process, but not the class attribute boost.
Initial Proficiencies
Source GM Core pg. 84
A level 0 character is trained in Perception, all saving throws, unarmed attacks, unarmored defense, and one simple weapon of their choice. Additionally, they're trained in a number of skills equal to 2 + their Intelligence modifier. The proficiency bonus for a level 0 character works the same way as normal, but since the level is 0, the total proficiency bonus for being trained is +2.
Hit Points
Source GM Core pg. 84
A level 0 character adds their Constitution modifier to their ancestry Hit Points to determine their starting Hit Points.
Apprentice Option
Source GM Core pg. 84
If the story you want to tell is about characters who have started training to become a particular class, you can grant them a small number of additional abilities. An apprentice character is trained in the skill or skills specified for their chosen class (such as Athletics and Intimidation for a soldier) in addition to the skills they gain through their initial proficiencies. They also gain benefits based on the class.
Solarian: An apprentice solarian gains a solar flare, solar nimbus, or solar weapon (but not all three). They can only select one trait for their solar weapon and don't gain attunement, so they don't gain the additional benefits of being attuned.
Other Martial Class: An apprentice of another martial class (envoy, operative, and soldier, for example) is trained in light armor, all simple weapons, and one martial weapon listed in the class's initial proficiencies. If a martial class not listed here lacks light armor or martial weapon training, give it a different ability as well.
Spellcaster: An apprentice spellcaster is trained in the appropriate magic tradition and gains two cantrips from their class. A prepared caster can't change these cantrips each day.
Solarian: An apprentice solarian gains a solar flare, solar nimbus, or solar weapon (but not all three). They can only select one trait for their solar weapon and don't gain attunement, so they don't gain the additional benefits of being attuned.
Other Martial Class: An apprentice of another martial class (envoy, operative, and soldier, for example) is trained in light armor, all simple weapons, and one martial weapon listed in the class's initial proficiencies. If a martial class not listed here lacks light armor or martial weapon training, give it a different ability as well.
Spellcaster: An apprentice spellcaster is trained in the appropriate magic tradition and gains two cantrips from their class. A prepared caster can't change these cantrips each day.
Gameplay
Source GM Core pg. 84
Combat can be especially dangerous for level 0 characters. For safety's sake, you might treat the characters as level –1 when determining what combat encounters are appropriate. For skill checks, they can still accomplish tasks with a simple trained DC using their trained skills, but success is less certain. Since they have fewer skills, the party might not have anyone trained for a given task. If you're playing these characters for more than a few sessions, consider advancing them to 1st level using the fast advancement speed (800 XP). If your group wants a longer experience at level 0, start the group without the apprentice benefits, then level up to apprentice (gaining those benefits and the apprentice adjustments for their class), and then level up to 1st level.
Treasure
Source GM Core pg. 85
As the characters start with 50 credits, their adventures up to 1st level should account for the rest of a 1st-level character's starting money. That means you'll distribute treasure worth 100 credits × the number of PCs, a large percentage of which should be in credits.