over 3 years ago - Sleipnir - Direct link
There are a couple methods:
a) Harvest naturally growing, mature plants around your settlement.
b) Start growing your own crops and harvest these. Make sure to put a Farm room down to auto harvest/plant.
c) Tame wild animals and either butcher them for meat or take care of them to get resources like milk.
d) Start fishing at lakes/ponds. Make sure to give someone with the fishing skill active a fishing rod. Then assign a fishing job next to the water with fish in it.

Later on you'll want to make things like flour because it doesn't decay like most other foods and can get you through winter.

Hope that helps!