Old-ish thread, but...
0) Look up your TDEE
here. This is the amount of calories you need to eat to maintain your weight. Eating less than that will lose weight, and more will gain. Each pound is worth about 3500 calories. Never go under 1200 calories each day, as it's difficult to get the right amount of nutrition at that point. Also, talk to your doctor about your diet before starting, if possible.
1) Mushrooms have barely any calories and are very filling. When we have them around, I mix them into everything. You can even make low-cal sandwiches with them (my favorite is white meat chicken + mushroom + a slice of swiss).
2) Fiber is really filling. If you can find a shredded wheat cereal brand that you enjoy & doesn't break the bank, stock up on it. Having that as breakfast will keep you satiated for a good portion of the day.
3) Chicken and peanut-butter are great sources of protein. White meat chicken is low-cal, but peanut butter is mid/high-cal. You can have the peanut butter straight or with celery (which is very low in calories) to even it out.
4) Yes, make sure you're getting enough vitamins, minerals, and protein / fat / carbs, as this can be difficult when eating few calories. An easy solution is adding meal replacement shakes and multi-vitamins into your diet. If you can't afford that, research the daily vitamin/etc. requirements & try to work your meals around them instead of just the calorie count.
