Answer for: A) Download the beep test app and continue to improve your level, you need to do alot of stop/start style cardio like Rowing machine, hill sprints, time trial running with 100, 200, 400 sprints in a certain time B) at 15 you shouldn't be doing weights as it can affect your growth, start weights at about 17/18yrs for arms do boxing workouts, for legs you can do strides and one legged squats and rabbit jumps over a bar at height and core do planking and sit-ups, also throw in burpees.you should purchase coloured resistance bands which can help with building strength C) give your local pro or semi pro club a call or contact a personal traininer
For stamina and speed do wind sprints. jogging only makes you a good jogger. doing 20-30 wind sprints for 10-20 seconds (depending on your present state of your body) will help you get faster and build stamina. For your core just do various ab/core work outs things like crunches, sit-ups, planks, v-sits ect(you can find a billion of ab/core work outs online). Getting your arms and legs stronger- do things like curls and push-ups and free weight and bench-press wont hurt. for legs your gonna need a gym or home gym equipment and just use the machines for legs and also leg presses/squats
