Start by getting a rewards card that includes what I like to call the magic categories; supermarkets, drugstores, and office supply stores. This holds true even for people who never buy groceries, have no need for office supplies, and rarely shop at drugstores. How can that be possible? Because these three places often sell gift cards for other places, too.
Among the cards to consider: American Express Blue Cash (or Blue Cash Preferred with a $75 annual fee); Chase Freedom or Ink business cards; and Discover It.