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.