I'm a bit confused..

Gradually increase your cals by 100-150 a week, starting at 1200. You will probably need to eat at 1750 for a while (6-8weeks) to get your metabolism up and going again. How much weight do you want to lose?

Eat nutrient dense food, lentils, nuts, nut butters, olive oils, brown rice, avocado to get your cals up if you are really struggling. Once you've been eating at 1750 for a while, you might start dropping weight again, or if youre like me, I had to cut back down to 1500 to start losing again.

How sure are you that you've entered all your calorie input? 900 seems scary low. MFP will warn you if you are under 1200 for the day. You don't want to eat under your goals as a habit or else you're body will go into the so called "starvation mode". Your profile says you want to lose 15 pounds. At what rate are you looking to lose? 1/2 pound a week, 1 pound, 2? If you're only taking in 900 calories, and your body needs more (with almost certainty it does), you'll feel it within days.

For me, I started at 260 at 5'11". I have a job that has me sitting most of the day, so I setup with sedentary, targeting 2 pounds a week. Initially it calculated my net calorie target at 1640, and then after a few weeks dropped to 1520. I tend to eat 100-300 calories for breakfast, then balance whether I want lunch or dinner to be the bigger meal. I've only been under 1200 calories in a day once that I can recall, but have been shedding pounds at an average rate of 3.75 per week (faster for the first 4 weeks as I was getting rid of alot of water weight). What I ate before I started is dramatically different then what I eat now.

I second FaerieCae's advice on the nutrient dense foods. You're body needs certain things to function 'normally'. Find what you like, and try different foods. Keep a watch on your intake ratio of carbs/fat/protein.

Hey, thanks for your response. I'm pretty sure that I'm entering it all, but I'll make sure. I'm looking at one pound per week.

So if I increase my calorie intake, will I gain weight?

Not if you exercise.

Hey, thanks for your response. I'm pretty sure that I'm entering it all, but I'll make sure. I'm looking at one pound per week.

So if I increase my calorie intake, will I gain weight?

Not if you're eating below maintenance, you will just lose slower if you have a smaller deficit.

Hi, it is really confusing when you first start isn't it?
If you really are eating only 900 calories a day you probably will gain at first then it will settle down and you will lose.
This is totally dependent on all the correct information being entered.

The most important thing is to establish whether you have entered all the correct information in MFP to begin with and to make sure you have calculated that 900 calories correctly. Is your height, etc correct? Did you enter sedentary or have you entered a certain amount of exercise; as this is allowed for by MFP in the calorie allowance. I always have mine on sedentary as any exercise I do then is a bonus of calories which I can eat or not eat as I choose. This is a whole other topic.

As an 18 year old male I would be very surprised if you are only eating and drinking 900 a day but , of course, it's possible.

Could you open your diary and show us an example of your day so we can check your calculations? Have you calculated your drinks, all of them. Some juices even have a very high caloric amount. Every almond, every cracker ,every bite if cheese all add up in a day. That one little biscuit could be another 120 calories.

I noticed someone said if you exercise you will lose weight but that is not entirely true.
MFP is set up so if you enter sedentary it gives you a recommended calorie allowance as if you were doing no exercise at all. You should lose weight on that even without exercise.
Remember losing weight is mostly about what you eat, exercise is more for your health as a general rule.

So, check your entry details and open your diary. We may be able to give more informed advice then.
Good luck.