The trick (besides discipline) is not setting ridiculous targets…people take their time gaining weight, so losing it and getting in shape takes just as much time. Also don’t use the scale as a measurement of success, it can be disappointing, but use the mirror and listen to your body. It’s more honest.
I use to weigh 210 at age 22, I figured if I would not start exercising I stay like that the rest of my life. Now I’m 35 and weigh 154 for some years now, have an athletic body and feel much better than I did when I was younger. I began all those years back with road cycling to lose weight. First I did a couple of miles before getting out of breath, each time a little further now I cycle 50 miles for fun. (cycling is less stressful for kneecaps and the back then running) gradually I also started training at the gym.
I figured out that listening to your body is essential, workout routines often claim to be the best, but in reality each body is different and reacts differently on workouts. Genetics play a big role some people gain muscle weight much more and faster than others. For me it takes longer, I used shakes of all kind and they had no impact on me which was frustrating. Now I know my body reacts best on more repetitions with lighter weights (for instance 30 reps with 20 kilo v.s. 8 reps with 40 kilo) and my body gets used to routines very fast, so each month I have to change my schedule and workouts drastically to trick my body.
I did P90X for a while, it’s fun and some exercises are brutal, but after a while it’s a bit tedious. However I do like and recommend the simplicity of the exercises, which could be done at home with just a couple of dumbbell weights. For a beginner I don’t recommend P90X, too hard and if you don’t finish the reps it can set you back. It’s also more for agility then losing weight.
But most important is nutrition, I’m not a calorie counter and I like to eat when I’m at a party. The trick is not to eat more than required each normal day. A body does not need much food, but genetically the human body still thinks it has to eat with every opportunity it gets (which was useful in prehistoric times when we had to rely on catching our own food) take smaller portions and stay away from junk food.
So, good luck with all those new year’s resolution and set small targets!