My family footprint right now is pretty large, especially because my mom and dad live in separate houses. As a family we own four cars, and both houses are far away from public transportation or grocery stores. We also eat a decent amount of red meat and often leave lights on overnight.
If I had my own house, I would first reduce the number of cars I owned to just one, and have it be a car that is either electric or biodiesel. In addition, I would want my house to be walking distance from public transportation or closer to school/work. In terms of food, I would cut down the amount of red meat and substitute that with leaner options like chicken, or completely eliminate meat some days and practice a "meatless Monday" of some sort. I would also buy more groceries from farmers markets or local producers so that way I know where exactly my food is coming from and know that my food hasn't traveled extremely far to get to my plate.
At my dad's house, we already have a tankless water heater, and I would keep this because it's more energy efficient than a conventional tank water  heater. Finally, I would invest in solar panels to install on my roof as the main source of energy in my home, as well as energy-saving light bulbs in lamps and ceiling lights. At the end of the day, I'd do a sweep around the house and unplug all of the lights that won't be in use.

Theres a lot of things id want to fix, but the main one would be how much food we waste and the amount of electrical energy we consume