Here are some very straightforward principles:
* Closer is better than farther. When you buy local, you minimize the transportation footprint of your food and also the deforestation pressure in tropical countries. You also support the local economy because farmers spend money nearby.
* Seasonal is better than off-season. This reduces the transportation footprint and the storage footprint, while maximizing flavor and nutrients. However, locally preserved foods have a small footprint too. Some CSAs have a canning club and then sell the canned goods in their farmstore: totally fine.
* Organic is better than conventional. This minimizes the use of toxic agrochemicals and supports soil health. There are other programs too, like permaculture and biodynamics, that generally take care of the Earth while raising food.
* Unprocessed is better than minimally processed is better than ultraprocessed. The less that's done to a food, the less resources it uses up before hitting your plate. The biggest difference is at the far end, though. Plenty of minimally processed items, especially those made by small local providers, use only a modest amount of resources.
* Plants are better than animals. There's a huge energy falloff between plants, herbivores, and carnivores/omnivores. Eating a lot of meat uses a LOT of resources. However, in a mixed farm or homestead, animals provide valuable cyclic services: chickens can eat kitchen scraps and insects, then lay eggs, and when the hen is done laying you can eat her too. Another trick is to serve meat dispersed through a dish, as in ham-and-beans, rather than in big hunks. Finally, use everything but the squeal! Modern processing is wasteful, but doesn't have to be.
* If you can grow at least some of your own food, do that. It's better than most other options because it uses a minimum of resources to reach your plate. Or your mouth, if you're like me. "Oh the mulberries are ripe!" *nom nom nom* (20 minutes later) "I should probably pick some to freeze."
Happily, what's good for the planet is good for humans. Eating fresh local foods, and more plants than meat, adds up to a healthier diet than the American tendency toward imported, highly processed food with too many animal products. That doesn't mean you have to eliminate anything, just think about the proportions of what you choose to eat and how it affects the Earth.