To determine an ability modifier without consulting the table, subtract 10 from the ability score and then divide the total by 2 (round down). Because ability modifiers affect almost every Attack roll, ability check, and saving throw, ability modifiers come up in play more often than their associated scores.

How are abilities calculated?

An ability score is determined by rolling 4d6 and discard the lowest score. This is repeated six times and each score is placed next to the ability where they are wanted by the player. This method tends to produce higher than average scores, which is more appropriate for a heroic character.

Where is the ability modifier on character sheet?

Your Spellcasting Ability modifier is a number taken from the ability used to power the spells. So for a Sorcerer, their Spellcasting Ability modifier is the same as their Charisma modifier found on the first page of the character sheet.