A.Z.
Dear T.,
I agree with most of the advice given already and I would like to add a few comments.
Look at where the food is coming from. If it is coming from halfway around the world it is usually grown so it can travel well at the expense of taste and often nutritional value. Food that has to travel many miles is also using more fuel and energy to get to us which is something to think about. Choose foods that are in season. Blueberries in winter come from the southern hemisfere...
There are a few foods you should by organic even if they are not certified organic which is a certificate that is expensive for small farms to obtain. It's best if you know who is growing the food.
Grapes and carrots and usually most veggies that you eat whole should be pesticide free. Grapes will store the chemicals more easily than other fruits.
If you are interested in learning more I would recommend Barbara Kingsolver's book "Animal, Vegetable, Miracle". It can be a little preachy but if you can ignore that, it is very educational.
I hope this helps,
A.