Many people find breakfast is the hardest meal to think of what to have but there are quite a few options which are good low carb things. Greek yoghurt and berries and seeds with a very small portion of a low sugar cereal like All Bran or granola. Eggs in any form with mushrooms, tomatoes or bacon if you eat that, some people can tolerate 1 slice of toast with it. Cooked meat and cheese are other things.
Everybody is individual in what portion size they can tolerate of the high carb foods, those being potatoes, rice, pasta, bread, pastry as well as cakes and biscuits, crisps, tropical fruits, starchy veg and things with added sugar.
Basing meals on meat, fish, eggs, full fat dairy, vegetables and salads and fruits like berries and limited amount of apples and pears with only small amount of those higher carb foods.
The book or app Carbs and Cals is a very useful source of the TOTAL carb value of lots of foods and meals. People do use other apps as well and get information from the back (usually) of packets and the internet.
People find substitutions for some of the high carb foods. Looking for Low carb recipes in the Food and recipes forum will give you some ideas.
The sweetest thing I have ever had in my life were Koeksusters make by my other half's sister.
Do have a look at the Learning zone (orange tab at the top) and come back with any questions.
A bit of information about what your HbA1C is and if you are on any medication will help people to tailor any suggestions.
One way to know what portions and foods you can tolerate is home testing with a blood glucose monitor which you would most likely to have to self fund but there are inexpensive ones available on -line.