I want to become a boxer. In order to start training I need to slim down but also gain strength, any advice?
It's all in the diet. More protein, less carbs. You need to eat less calories than you burn to lose fat, but if you are trying to gain muscle you don't want to have a calorie deficit. So, eat enough protein daily, and just the amount of food you need to maintain your weight + need for exercise. Then after you gain the muscle, you can easily restrict your calorie intake about 10% and you will lose fat daily. It may take a week to lose a pound, but consistency is the key. If you can't maintain a disciplined diet where you cut out all sugar carbs and carb heavy foods then you'll have a hard time.