Eat low on the food chain. On average, it takes nearly 10 times as much fossil fuel to produce animal protein (including commercially caught or farmed fish) compared to plant protein, like beans and grains. According to a 2006 University of Chicago study, a vegan diet contributes 1.5 fewer tons of CO2 or CO2 equivalents to the atmosphere each year than the average North American diet.
Buy local produce when you go to the grocery store rather than items trucked in from far away, and bring it home in reusable bags. There can be exceptions, as when imported food has been produced more sustainably than local and shipped (energy efficient) rather than flown (very energy inefficient).
Buy sustainably or organically produced food. Conventional farming uses massive quantities of petrochemicals in the manufacture of artificial pesticides and fertilizers and to run farm machinery. Organic and other traditional or natural farmers use minimal fossil fuel inputs. Some even use draught animals instead of internal combustion engines. Look out for certified organic products. There are several bodies who certify products as organic such as the Soil Association in the UK.
Buy sustainably harvested seafood.Seafood from either fished or farmed sources can exist over the long-term without compromising species' survival or the health of the surrounding ecosystem, making it a sustainable source of food. Seafood Watch
Eat less meat. More land has to be put into agricultural production to produce meat than to produce plant products. This means less "nature" for us to play in and wildlife to live in.