A while ago I hit 300 pounds. I'm 18, male, 6Ƌ", so 300 pounds isn't nearly as bad for me as it would be for the average person. I have huge bones and a huge body frame, I look like I play football. But I needed to lose weight. So I did. I adopted a healthy lifestyle of eating. I had been going to the gym a ton but no diet whatsoever. I've since lost 15-20 pounds. I started the week after Thanksgiving. I think considering I did this through the holiday season and we've had so much good food around the house, I think I've done okay. But my goal is to lose 50 pounds, then reevaluate. I'd like to hit this goal by May, June at the very latest. And I need help doing it. This is my current routine.
I'm a senior in high school, but I'm technically home schooled so although I take outside classes, I don't have regular phys ed. I make it to the gym a couple times a week. I spend at least an hour there, making at least half that time cardio. For the 2nd half, I simply focus on strength training. I try to change it up every week, so I'm always working different muscles, and I switch between the treadmill, bike, and crosstrainer. For dieting, well..it's pretty straightforward. Healthy breakfast, with coffee and a cup of juice, cereal, whole grain waffles, oatmeal, or eggs. I'll also have a whole grain english muffin to go along with it.For lunch, I'll eat a healthy microwavable meal, or I'll broil a sandwich with healthy sandwich meats/cheeses. For a snack I'll usually have some whole grain crackers. I'm a bit more lenient on myself at dinner but my mom has made it a point to make healthy meals for me, and I'll have a 1 or 2 helpings of salad before I even have any of the main course. I don't eat anything after dinner. Other things: I don't drink soda, and the only fast food I eat is Subway, Jimmy Johns, or the VERY occasional Chick-Fil-A. (because come on, it's chick-fil-a >.>) I don't eat burgers, pizza, or anything fried.
I've been going multiple times weekly to the gym since last Summer, and I don't take it easy on myself. I'm not a beginner, though I have a lot of weight yet to lose. To keep with the weight loss, I have to focus. And with work, school, and college applications, it's been pretty crazy for me. Last week I gained a pound for the first time since November, and I'm afraid I'm slowing down, even though I'm not sure how it could've happened. How can I step it up?

Find more things healthy to eat that you like. I was a big meat eater, but I traded the steaks for turkey breast and shrimp! Its all mental, you have to stay busy!

The thing you need to focus on is increasing your metabolism. Your metabolism basically is how quickly your body uses energy i.e. burns fat. Your body is burning fat all the time, even when you're sitting down! How much energy your body uses however depends on your metabolism.
You can increase your metabolism in a number of ways:
Weight Training - weight training is a very good way not only to build muscular definition and lean muscle mass, but also increases your metabolism. Your muscles require energy to function, and as such use up energy that your body either stores or consumes. Therefore, your body literally burns fat in order to maintain muscle mass. As such, more muscle mass requires more energy, and in thus increases your metabolism. Also, when you weight train, you can eat meat! The protein from the meat is used to build your muscle mass after a workout, rather than storing into your body.
Eating Properly - I'm sure you know the difference between healthy food and bad food, so I'm not going to go into that. What you probably do however is eat three large square meals a day. If you want to really increase your metabolism, you should eat 5 or 6 SMALL meals. Basically, eat the same amount as normal, but spread it over the day. Basically, by eating only three large meals, your body signifies the long periods that you go without food as a lack of nourishment or a food source. Your body then inadvertently stores more energy/fat during this time, rather than expending it. If you eat within every 2 to 3 hours however, your body recognises that there is a constant supply of energy, and will thus relieve itself of the energy it stores.
Additionally to this I should signify the importance of breakfast. People who skip breakfast or eat breakfast too late trigger this same 'storage' response in their bodies, causing all energy they consume or have consumed to stock up.
Drink a lot of water - You should drink at least 8 glasses a day. Don't only just drink when you feel thirsty! Preferably, the water should be cold. Drinking a cold glass of water or two in the morning helps to kick start your metabolism for the day.
On a final point, I should add the best way to exercise when it comes to fat loss. Obviously, you will want to be aiming for cardio workouts, such as running, boxing, swimming, or bike riding. What people do wrong however is perform the exercises lightly at a mild pace for a long period of time, say 50 minutes. In actuality, if you want to lose fat, the best exercise routine shouldn't be for any more than 20 minutes! The trick however is that the workout must be Intense. You need to be absolutely exhausted by the end of the workout, sweating all over. Start slowly for a couple of minutes. Then over a four minute period gradually increase intensity to a maximum point. After 4 or 5 minutes, go back to light intensity and repeat this 4 times. This should only take 20 minutes.

50 pounds by May isn't possible, by 30 is very reasonable.
Keep your daily calories at 2k.
Let's start off with your breakfast... cut out the cereal and whole grain waffles. Yeah, they're whole grain, but they're not still not good for you. Cereal is processed and contains a lot of fats and sugars. Oatmeal or eggs with coffee is your best bet for breakfast.
Most microwavable meals aren't healthy, stick to lean chicken, tuna, sweet potatoes, fruits, veggies, etc.
As for snacks, they really aren't necessary, but if you're REALLY hungry, then you should eat an a fruit or some veggies instead. They don't taste as good, but if you're serious about losing weight then fruits and veggies will be your best friend.
Now, your exercising. You should be going to the gym EVERYDAY. When you work on cardio, don't cheat yourself and go on at a slower pace, give it all you got and make yourself feel that burn than burn is what you want when you're trying to burn fat. Lift weights everyday too, lifting is actually proven to be those effective way to burn calories, but you have to lift HEAVY! Don't work the same muscles everyday, workout a different muscle group everyday. When you get back from the gym, you will probably feel very hungry. All that junk food will be very tempting, but don't do it! Eat something healthy and drink loads of water.
Stay determined and motivated man, right when you get home from school, put on your workout clothes and hit the gym and keep your energy flowing!