Is vous plural or singular or does it not work like that? Or is it like something else

Any french speakers? i need some . could someone check the grammar of my french translations? 1. i would get a card for a bookstore. > > > *je voudrais acheter une carte-cadeau pour la librairie en le centre commercial.*2. they would have to go to the bookstore, the jewelry store, and flower shop. > > > ils devraient aller à la librairie, la bijouterie et le fleuriste.3. a good amount to spend would have to be a few hundred dollars. > > > un bon montant à devrait être de quelques centaines de dollars.4. i decide what present i am going to get for a person by thinking about what would be important to them. > > > je décide quel cadeau je vais chercher pour une personne (by thinking about) ce que cela important pour leur.for the 4th question, how would i say "by thinking about" in french? and by the way, i need the french sentences checked, those are what i wrote.