What was the initial source of energy? According to law of conservation of energy;
"Energy can neither be created nor be destroyed but may change it's form"
Where did the energy come from which led to the creation of universe by big bang?
 A: Well, if you want an answer at the 9-grader level, it's probably this:
We don't know, and it's mostly irrelevant to how the universe behaves now. In particular, the Big Bang Theory doesn't care about what happened before the Big Bang. According to many interpretations of different branches of physics, the question doesn't even make sense, e.g. what happened before time existed? Well... nothing. How could it? Time didn't exist. Things don't happen without time.
Asking the Big Bang Theory to explain how the universe came to being is like asking evolution theory to explain particle physics. It's just outside of the scope of the theory.
Now, there's a lot of hypotheses that try to explain it anyway. However, the problem is that we can only use behaviours we observe now - e.g. we can observe nowadays that there are some asymmetries in the universe. For example, anti-matter and matter are mostly symmetric, and when they come in contact, they annihilate each other. So... why is almost all of the visible universe formed out of matter?  People trying to gain insight into issues like this will gain more evidence to theories about how the universe formed in the first place (within the general assumption that the fundamental laws of the universe do not change with time).
Finally, it should be noted that we don't actually know if we know any fundamental law. It's not even something that's decidable from within the universe, we just get closer and closer to reality with our models. For example, thermodynamics is usually considered as fundamental, and thermodynamics says (extremely simplified) that the entropy of a closed system will equalize over time: for example, if you put hot water and cold water in the same container, they will eventually mix to form a body of warm water. However, this is actually only a statistical effect, so when you reach maximum entropy, you will get localised pockets of lower entropy (e.g. some parts of the container have water warmer than average, and some colder). In fact, there is a hypothesis that tries to explain why our universe doesn't have maximum entropy yet, based on this behaviour - the idea is that our universe is a tiny pocket of locally low entropy inside the "real" universe, which is close to maximum entropy.
A: The simplest way to look at this is that energy can also reveal itself in negative forms. Don't think of it as something only positive, but also there's a negative part of it in the universe that's not directly visible. 
For example, we have good reasons to believe that the total energy in the universe adds up to zero. We also have experiments that show that energy is not conserved in very small scales. For example, beta decays produce particles (W-boson) that are about 80 times heavier than the constituents for a short time.

This is OK because of the Heisenberg uncertainty principle, which trades energy for short time periods. In addition to this, we also have good models for quantum chromodynamics (quark physics) that shows that vacuum energy fluctuates and is not constant. Also above that all, we know that gravity only attracts!! Why is that the case? Some scientists see this as only negative energy, unlike electromagnetic and other forces.
So you see, the conservation of energy is not that strict + we have lots of evidence showing that a zero energy state is not a stable state at all. From that point, we cannot really say it's impossible that energy is born by itself in the universe, but only say it's probable.
Note: This is a fairly complicated topic and you need to do physics for very long years to learn all this stuff. So be ready to do that, even if you're not a physicist. I'm avoiding going into details like why energy would not remain around zero. Short answer: CP violation. The universe is not 100% symmetric in time.
