I'm 15 and I'm super skinny. I'm about 5 feet 8 inches tall and My weight is about 4o k.g. I'm so weak. What exercise should I do and what should I eat?
I would suggest you start with the basic compound movements - deadlift, squat, bench press, overhead press, dumbell row, pull up, dips, push ups.
For starters learn proper form on each of these exercises, then I can suggest you the Stronglifts 5x5
It is good for beginners and you will see progress quickly.
Regarding the food - start eating 3-4-5 times a day and try to have protein and carbs in every meal. Eat mainly meat, eggs, fish, vegetables, fruits, rice, pasta, potatoes and drink milk. When you stop progressing, just try to eat a little more. Don't avoid fats completely - eat nuts, use some oils, but don't spill yourself