Start off by taking your morning body temperature as soon as you wake up.
Then also take it randomly throughout the day up until you go to bed.
Next, take some online quizzes to find your dosha - ayurvedic constitutional body type.
Do you have an illness with a label?
Have you tried gluten free?
Guesstimate of your average daily calorie intake?
Proportion of macronutrients? (protein/fat/carbohydrate)
If you want to get serious too, then get a hair mineral test and find out what's going on. Scio/epfx testing is also good if you can find someone locally.
Usually I see when people are out of balance they are eating the wrong ratios of macronutrients in relation to their body type or current imbalance. You might need to temporarily go on a high fat, high protein or high carbohydrate diet to rebalance. Or not, just depends on the situation.
It's a given your thyroid system is out, but first take the tests to see what your diet should be like to support whatever comes next like Iodine /selenium supplementation, aminos, more carbs, cleanses, etc.
If your adrenals are out then you need to go one way. Some people can gain weight while exercising if their adrenals are fried. Conversely if you have an extreme kapha excess you will need to exercise and eat light.
When the liver is is severely congested it is very hard to lose weight. Also it depends a lot on what type of weight imbalance it is, water/edema, lymph, fat, protein, sluggish metabolism, etc.
Blood Type Diet is good to *look* into like WS recommended but I do not agree with so many diet restrictions. If you have to jump through so many hoops with your diet there are deeper problems that can be addressed.
You need to press the right buttons in the right sequence to solve the problem.