Nutrition plays a main role in ensuring that your Pals are happy and healthy. Early on when you first begin, your primary source of food will likely be Red Berries.
They are easily accessible within a few steps from the starting area, and they can be planted and harvested shortly into your playthrough.

However, once you progress a bit further, you'll want to upgrade to a more nutritious supply. And given where you started, Baked Berries are the obvious transition.
They have a higher Nutrition value than their raw form, and can be made easily with just the Red Berries.

Later down the road, the Salad becomes the ideal choice for a significant Nutrition boost (84 compared to 21). With a more effective food, you won't have to worry about the quantity nearly as much.

Important Note
Speaking of quantity, make sure you're carefully managing your supply in order to avoid letting the food spoil. The Cooler Box (level 13) is a necessity once it's available, so be sure to keep a specific Pal assigned to it at all times.