Firstly you need to decide what you need to change and what is causing you problems. Eliminating foods from your diet is the first step and you can get an allergy test which may help find what is causing illness. Alternatively you can follow a FODMAP diet to help eliminate and find the source of the foods you may not be able to tolerate.
Generally the most common intolerances are Dairy, Gluten, Soy, but there several others such as E numbers and additives in foods. Do not confuse with an allergy, as with an allergy you cannot ever eat the food and could go into anaphylactic shock when eaten. With an intolerance, you can from time to time eat the foods as long as it is not in your diet every day or too regular.
For me, the problems started when I became intolerant to dairy. So to heal my stomach lining, I had to completely removed dairy from my diet for 3 months, this is to allow your body to fully recover. I was unable to tolerate chilli, spices, garlic and anything that would flare up my stomach, because I had damaged it from dairy.
A food intolerance is when your body cannot digest a food source, normally from the sugar proteins within the food item.
Once I had allowed my stomach to heal, I could tolerate the spices again. I can now also tolerate some dairy, but I mainly stick to lactose free or alternative vegan products.
The gut health is one of the biggest things that helps with a lot of issues, such as IBS, digestion, weight loss and even mental health.
Some good items to introduce to your diet, is pre-biotics and pro-biotics. I found when I had a damaged stomach, I took the pro-biotic tablets and they helped calm my stomach down. Now I have a much healthier diet, I do not need to take them all of the time. You can include Yoghurt (including lactose free) as a natural pro-biotic.
If you include pre-biotics in your regular diet, this will ensure your stomach is producing enough good bacteria, to allow the stomach to work as it should. Mainly foods such as sauerkraut, gherkins and foods in vinegar all assist. A regular addition of Apple Cider Vinegar (1 teaspoon added to water), a few times a day, can also assist with this.
Some signs of an intolerance are stomach upsets, bloating, wind, skin rashes and itching.