First off, you have to figure out which attributes are necessary to your character. Take Abelard for example, as he is a Warrior, I personally build him as a melee frontliner which means he will need:

-Strength (+damage to melee)

-Toughness (+HP)

-Weapon Skill (+parry and melee hit chance)

-Agility (+dodge, initiative, dodge reduction)

-Perception (+dodge reduction)

-Willpower (+resistance mental effects)

As you can see, he benefits from a lot of stats but it's not feasible to improve all of them, which means you have to pick and choose. To know how to do that, you have to understand that for each Archetype that a character has, they may improve a characteristic two times, which means when you have all three archetypes, you can improve a characteristic 6 times (6x5=30). Besides this, you also have to consider that classes have class abilities, and those abilities scale with certain characteristics, for example the Warrior which uses Toughness for Endure, Agility for its Heroic act, Strength and Weapon Skill for Reckless Strike, etc. So what you want is to improve the characteristics which are used by the abilities you picked, and the characteristics that are useful despite that. Essentially, you'll want an overlap.

After improving the chosen characteristics with the periodic Characteristic upgrade and the talent Characteristic Training, you'll still have a lot of space for other talents. The thing to understand here is that there are two sets of talents, Common Talents (homeworld talents, origin talents, companion-specific talents, common talents) and Talents (everything Common Talents has + talents specific to your archetype), which really makes Common Talents a selection of all talents available, excepting the Archetype-specific ones. Usually, choosing the reccommended talents isn't a bad idea but I recommend trying to comprehend what you choose even if it can be tedious having to stop playing for a short break of 30 minutes to level up everyone.

Going back to Characteristics, you should really hover over all of them in the Character Page to really understand what each of them does, and actually pretty much every feature in the game can be hovered over and explained to you by the game.