The Attack Roll

When making an attack, you roll the following:

1d20 + Ability Score + Proficiency - Size Modifier

Ability Score

The Ability Score used in the attack depends on the type of attack:

For weapon attacks, the Ability Score is either Might or Agility. Might is used for Forceful weapons and Agility is used for Finesse weapons.
For spell attacks, the Ability Score is the based on the Spellcasting feature of the class the spell comes from. For example, Masters of the Hidden Form use Wits to for their spell attacks, while Heralds use Resolve.

The Proficiency used in the attack depends on the type of attack:

For weapon attacks, you add Proficiency if you have the Combat Proficiency Feat.
For spell attacks, you add Proficiency if you have at least one level in the class the spell comes from.

The Size Modifier in the attack depends on the type of attack:

For weapon attacks, your Size affects how easy it is to hit your target. Your Size Modifier is subtracted from your roll.
For spell attacks, Size has no effect on the attack roll.