For a new player, this is a very popular question. For each of the Investigators names (Dexter, Amanda, Bob, and Monterey), whenever you draw a card from the respective deck (Spells for Dexter; Skills for Amanda; etc.), you draw an extra one, but you still select only one of those drawn. For instance, Bob's at the General Store and decides to take advantage of the Location Ability. Any other player would draw three cards from the Common Item deck and select one for purchase. As Bob has the unique ability to draw an extra card, he draws four, but still only selects one for purchase. Here's another example. Dexter, during an Arkham Encounter, passes a check and may draw a Spell from the Spell deck. Due to his unique ability, he may draw another one and select between the two. Finally, this special ability works during set-up. Thus, As Amanda starts with two Skills, she draws three and selects two of them at the start of the game.
Hope that helps.