That's really not how magick works kittykat, and I think some could argue that doing spells like that can be dangerous since they could backfire or turn out not quite how you imagined.
Spells I would suggest are spells that help you stay healthy and energized, maybe use some sigils in your food/drinks, for example stirring your tea in a specific motion, or when you're cooking something.
But also make sure you are staying safe, magick and other stuff can help you move along with the diet you want but weight loss can be dangerous and stomachs aren't naturally flat. Just try your best to stay healthy and active, throw a little bit of magick in there if you want and your body should respond over time.
This might be out of line a bit but I'd look into some self love/beauty type spells! Stuff like that can help you get along in a number of different ways.
while i worry it could cause you to fixate on your image, you could try a vision board with pictures of the body you're aiming for and some positive quotes to keep you motivated. [you could also fill it out with cute/funny comics about working out] take a few minutes every day to look at the board, then close your eyes and visualize all the hard work you will put in and your end result. [you might wish to chant something that inspires you] just be careful not to fixate on being thin but being fit [it can be a dangerous cycle to fall into] i also suggest a fun work out mix to keep you pumped, also, try signing up for classes, you can meet like-minded friends and it can keep you motivated.
Good teas to help aid you in weight loss are green tea and mint. You still have to eat well and do excercise but this can help a little too. You are also supposed to drink five to eight cups of water a day which is roughly a gallon. When preparing meals you can make it fun by adding some subtle kitchen witchery to your cooking! Maybe that will make cooking more fun for you.