Sorry in advance for the novel, I didn't realize how long this was till after I had typed it all out...
Welcome and way to go on already starting some healthier habits. I think the approach you're taking of incorporating regular, healthier foods is fantastic. I know strict diets work wonders for some people but I find they're so hard to maintain and I usually get fed up with being so restricted and only eating certain provided foods that I end up chucking it and go back to eating like crap and gaining. If I want a cup of frozen yogurt I'll have a cup of frozen yogurt, but I really have to think would I rather spend 400 of my 1200 daily calories on like 5 other items that are more nutritionally balanced and will provide longer satisfaction and satiety or the ice cream. Sometimes I pick ice cream, sometimes I don't. I'm not doing anything special except counting calories and find that I can eat more if I pick healthier options and know that works for many of us on here. Moderation is key for me.
As far as how many calories you should be consuming check out the link provided above or you can google it and figure out a deficit from there but definitely don't go below 1200 a day is common advice; it's very difficult to get all the nutrients your body needs on a lower number. You can sign up for a free website like lose it or myfitnesspal which also have smart phone apps that will help you calculate how many cals you should be consuming to lose weight and will track calories and adjust based on weight loss and goals. They also will track your macronutrients and on the phone both have a barcode scanner which I find useful as well as a vast food database.
As far as fiber goes you should aim to get at the bare minimum 20g a day in but should aim higher; I myself usually have a range of 25-35 grams and feel like 25 is a good minimal amount. Protein is important to help maintain muscle as you lose weight and the us government recommends minimally about 46 g of protein for women but you should eat more if you can. I'm a vegetarian and really have to work to hit my protein intake and minimally will eat 60 g but have been known to get up to 85 or 90 and those who eat a lot of meat or do atkins type programs will have much higher protein intake. High protein foods will also help keep you fuller longer.
Stay hydrated! Water is very important for your body to function properly and dehydration can have serious impact on your electrolytes, make you constipated the list goes on. Drink your water
And I know you said you only eat one meal a day. If that's what works for you that's what works for you. At the end of the day it's calories in versus calories out and whether those calories are spread out or consumed all at once really shouldn't impact overall loss. BUT I will say if you're going a long time without eating and you haven't been paying attention to what you're consuming prior it is possible since you have not eaten in such a long time and you're ravenous that you are way over consuming on calories. I like eating small meals and snacks throughout the day, other people do intermittent fasting and only eat during certain hours like you have been doing. Just use trial and error and see what works for you. As long as you're consuming in your calorie range that you set your weight will come off.
Invest in a good set of measuring spoons/cups, a food scale is really helpful (and can be found for less than $20), and if you bite it write it. You may find you were consuming more than you thought. Good luck and let us know how it goes!