As a number of other people have pointed out, the prevailing opinion on what is good and what is bad changes regularly. I'm inclined to go with Michael Pollan's advice of "Eat food. Not too much. Mostly plants." as a good place to start. Skip most processed foods (ie, anything in packaging). Also pass on refined sugars (HFCS, etc). Drink lots of water instead of soda. Eat fresh fruit, vegetables, lean meats, eggs, etc. If you live in an area with a farmer's market, take advantage of it. Learning to prepare food for yourself is very rewarding.
