Each Ability Score is either Strongly linked to two others, or Strongly linked to one and Weakly linked to two others.
Strength: Strongly linked to Constitution and Dexterity
Charisma: Strongly linked to Intelligence and Wisdom.
Constitution and Dexterity are Weakly linked.
Intelligence and Wisdom are Weakly linked.
Constitution and Wisdom are Weakly linked.
Dexterity and Intelligence are Weakly linked.
Whenever you take an action, it will specify a primary Ability Score - in addition, select a second ability score that fits with the action being taken.
Add the bonuses to those two ability scores together; if they are Strongly linked, add one. If they aren't linked at all, subtract one. That's your modifier for the action.
Creating a Character:
All your ability scores start at +0.
Pick two ability scores. If they are not linked, Weakly link them. If they are Weakly linked, they are Strongly linked instead.
Pick an ability score other than the one given by your class; it gains a +1 bonus.
Each class gives a +1 bonus to one ability score, as well as some special abilities.
Some classes "cost" your ability score link, forcing you to use it to connect two specific ability scores. For example, Paladins give a bonus to Charisma, and force you to Weakly link Charisma and Strength.