Getting some lean muscle will be your first order of business. If you haven't worked out much before, it should be pretty easy in the beginning. If you can make it to a gym 2 or 3 times a week, thats a good start. Start with a full body workout each session. One exercise per body part, and hit each muscle group every time (back, chest, legs, abs - arms if you want to, but do them last). Use compound movements (squats, rows, presses) since they work arms as well and use more stabilizers. Don't go to failure at first. Use a weight that you can do 10 reps comfortably, but struggle to get 15.
On days you dont lift (allow 24-48 hours between lifting workouts) do some kind of cardio. Run, jog, bike, rollerblade, jump rope, punching bag, swimming... anything that keeps your heart rate up for 30-60 minutes at a time. One day of total rest a week is good for the body and soul.
Diet: there are a lot of methods to reaching the same goals, but a few "rules" that must be followed. Calories are the primary factor in determining fat loss. You must have a caloric deficit (burning more than you are eating) or you will not lose fat. Conversely, if you eat too little, your body will slow its metabolism and hang onto fat, so you have to find the right range for your body. Gerneral rule of thumb is 10-12x bodyweight in calories each day (higher in the range if you're working out). If you can, divide those calories into 5 or 6 small meals to eat throughout the day (every 2-3 hours). It will keep you from being hungry and keep metabolism up so that you will use the nutrients and burn fat more efficiently. It can be kind of hard if you're in school, so little snacks or shakes you can have between classes might be very helpful for the sake of convenience.
Once that is in order, check the content of your diet. You need to be getting enough protein to build and maintain lean muscle mass (which keeps metabolism high) which is generally .8 to 1g per pound of body weight.
Carbs should be complex (oats, whole wheat breads, veggies). Avoid sugars and white foods when possible (white bread, white rice, pasta etc) because your body metabolizes them like it would sugar which increases likelihood of fat storage. Whole grain breads and brown rice and the like are burned more slowly and less likely to be stored. Vegetables are always a good choice, fruits are ok, but don't overdo it (sugar).
You must have fats. They are not the devil. But fat has 9 calories per gram (as opposed to 4 calories per gram for carbs and protein) so those calories can add up fast. A little healthy/unsaturated fat each day will help your body to use it's own fat for fuel (under a caloric deficit). As well as regulate hormones and various body functions. Go for nuts, natural nut butters, avocadoes, olives/olive oil, fatty fish etc.
Also good is to drink 1-3 litres of water a day.
Hope thats helpful. Good luck.