What's the best way to gain lean muscle mass, lose body fat and increase testosterone levels? What types of food / supplements recommended?

Well. No way to increase your testosterone . Eat healthy meals that are balanced in nutrition and exercise that is balanced with cardio and weights and strengthen your core. You might benefit from a personal trainer to get that six pack look you want. Supplements really do more harm than good. Healthy eating supplies your supplements!
Nutrition . Decide on most important, gaining muscle or fat loss? Reason is that the two processes work independently and against each other. Trying to lose fat will impede muscle gain and vice versa. Diet, whole foods 1st, supplement 2nd. When I'm gaining muscle I increase protein and carbs and some fat and when I'm cutting i lower carbs and do more cardio. If I don't reach goal, it's due to poor diet.