Buy Farming tech (level 2 before anything else) -- harvesting crops yields more food per basket than berries do (7 vs. 2). Then get level 3 as soon as you can after that -- fishing doesn't require any waiting time, unlike crops.
[Level 2 Science will help you get more tech points, so that's a good one to get after Level 2 Farming].

While you are harvesting crops/foraging for berries, use as many villagers as you can to farm (I sometimes leave one at the research table, sometimes not). Once the bush/field is empty, switch everyone back to the research table so you can get that next farming tech.

Try not to get distracted by building or breeding until you have a stable supply of around 400 food. And you might want to pause the game if you'll be away for a while -- switching villagers back and forth from farming to research requires a little babysitting.

If you get into real dire straits, hunt for mushrooms.