determining which foods to eat is beyond easy. the less ingredients a food has, generally the better it is for you. oatmeal is better for you in terms of nutrition and (for the sake of comparison) body composition than fruit loops, chicken is better than hot dogs, peanut butter is better than ice cream.
the hard(er) part is determining what amounts to eat.
a general guide for determining maintenence calories (an amount that will allow you to maintain your weight, ie. not gain or lose weight) is to take your lean bodyweight and multiply it times 15. To gain weight, eat more than that. To lose weight, eat less than that.
Very simplified, but gets the general idea across.