Hello Sarah,
They are tricky questions to answer in many ways. Before joining the forum here, I just mooched about book shops (new and second hand), and I would read reviews in newspapers, especially the weekend ones. Occasionally I would have a book or author recommended to me by a friend or family member, but to be honest I only trust the opinions of a couple of people and learned the hard way.
It might be the cover that appeals and the back cover blurb that pushes me to purchase, or the blurb alone. I may see an author being interviewed and think that I like what they have to say and would like to read their stuff.
Now I'm on the forum I definitely get ideas from the other members, and I have got to know the people who have similar tastes or who are so astute in their reviews I will know whether I would like a book / author or not. It can still be a bit hit and miss at times, but that's half the fun of it.