While I certainly believe that we can make decisions to lead healthier, better lives at ANY time of the year, January is kind of perfect for determining to actually make a step in that direction. It's a fresh start. Here are some steps to help succeed with your "resolution" (goal is a probably a better term?).
1. Be specific with your goal and make sure it is attainable. What do you want to accomplish? Maybe you want to drink more water. How much more? When is the best time to get it in? Try visualizing what it would look like to incorporate this into your day.
2. Find some motivation. Why do you want to do this? I think a good answer to this question (for anyone) is: because I want to be kinder to myself. Your body is an amazing asset; figure out ways to make it work for you as long as you can.
3. Tell your goal to someone else. Think of a time when you have succeeded--what got you there? Probably having accountability. Keep tabs on yourself (if you happen to have that kind of motivation); write it down in your calendar or make an alert on your phone to check in with yourself. Or have someone check in with you daily or weekly.