Losing weight quickly is not ideal, as it will usually pile back on quickly. A healthy amount of weight to lose per week is 1-2 pounds, with exercise at least 3 times per week, you could possibly lose up to 3 pounds weekly.
You need to eat correctly and not starve yourself, to ensure you are losing more bodyfat and not lean muscle tissue. Eat small regular meals throughout the day, reducing saturated fats and sugar.
Do a combination of cardiovascular and resistance training. It is important to do resistance training, as the more lean muscle you have, the more bodyfat will be used as fuel, doing weights will help to speed up your metabolism.
I don't know your sex, but you say you don't want to bulk up. If you are female, this is unlikely, as you will not have enough Testosterone to build muscle mass. If you are just looking for tone and definition, select manageable weights and higher reps, so look to do 2-3 sets of 12-15 repetitions of each exercise.