Choose goals that are attainable. Don't say I'm going to lose 20 pounds in 3 months. Although that can be done but it's not a very healthy route to take. 1-2 pounds per week is ideal, which would be about 10 pounds lost in 3 months. If you have not been working out- Don't go to the gym and over do it on day one. You will be left sore and miserable. Ease into fitness. If your new, plan to exercise for 30 minutes 2-3 days a week, and gradually increase as the weeks go by. Set goals for the week or month. Not too far out there that it seems impossible.
When trying to eat healthier, make small changes. Aim for glasses of water per day, prepare one new healthy recipe each week for the month of January, replace one can of Diet Coke with water somewhere in your day, eat a piece of fruit per day, add veggies with 1-2 meals in your day. Increase these changes as they become part of your daily routine and make them a habit.