Pre-diabetic, otherwise known as at risk of diabetes, means that your body has started to show some insulin resistance and is starting to struggle to get your blood glucose (or blood sugar if you prefer) down to a normal fasting level within the usual time after a meal.
The best way to help your body with this is to reduce your carbohydrate intake (all carbohydrates, not just sugar, as your body converts almost all carbohydrates into glucose eventually). Many of us who are already Type 2 keep total carbohydrates for the day under 130g but as you are pre- diabetic you may not need to go so low.
I would suggest keeping a detailed food diary of what you are eating for a few days, in particular recording how much carbohydrates are in the food that you're eating (or drinks if you drink things other than water and black tea/ coffee). Then you can get a picture for your current total average intake, and maybe consider dropping it by, say, a third. So maybe having smaller portions of bread, rice, pasta, potato etc and putting some more vegetables on your plate instead to help you feel full.
Losing weight if you are able to can also help to bring your blood glucose levels back in line, as a big part of insulin resistance is internal fat around the liver and pancreas (known as visceral fat). I appreciate this isn't always easy, but if you do substitute vegetables for carbohydrate-rich foods then this will usually reduce your calorie intake to some extent as well.
Has your GP (or whoever sent you a letter saying that you're pre- diabetic) suggested a time frame for rechecking your blood tests? The measure they will have used (HbA1c) allows an estimation of average blood glucose over the 3 months before the test, so if they test you in 3-4 months that should help you see if changes you make in the next couple of weeks are enough.