Blackberries are good berries to eat because they give Perception which will help you find more stuff. Blueberries (a forageable, they show up on your minimap) are good too, they give you Int.
If you mouse over the berry or nut it will tell you what attribute it gives.
I am contractually obligated to pimp out Sevenless' guide:
viewtopic.php?f=42&t=40714To eat the food, you right click on it, and it will say "Eat". If it doesn't say that--it's not edible.