Energy and Matter I was watching a show about the big bang theory.  They were saying that in the beginning all that existed was energy.  After the big bang that energy transformed into matter which then started forming into different elements.  How exactly does energy change into matter and at what point does this happen?
 A: Energy transforms into matter and vice-versa all of the time, according to Einsteins famous relation: $$E=mc^2$$
Essentially, if pairs of massless particles (say photons) have enough energy, they have a finite probability of transforming into two (or more) massive particles.
A: We are living in a material universe, and everything around inside this material universe is a matter, and (kinetic) energy is also nothing more than an impression of the existence of the matter. Other energies can be defined based on the purest form of energy which is kinetic. I don't know albeit if there we would need another building block beside the Kinetic energy or not to be able to build our universe in a bottom-up approach.
Consider matter as volume-less point moving at a velocity. The purest matter would be e.g. light particles and we have many electromagnetic and other particles which can all be the purest form of matter. Then each group of particles that fall in an structure would form higher levels of matter. The content of the kinetic energy of each can now be averaged to give other forms of energy, atomic energy if the structure is atom and chemical energies if the structure is molecule and etc. Therefore you may like to call a coherent energy (entrapped energy in a material structure) as matter (in the common view of matter).
A: Mass is nothing but most concentrated form of energy. As its the most concentrated form, all attributes of energy (possessing momentum, gravitational interaction etc) get magnified. You can see the equivalence in the popular equation $E=mc^2$.
Not just in the beginning, but even today all that exist is energy (and, Spacetime, other dimensions).
Energy can be converted from one form to another. That's what is done when mass is created. We do create mass on regular basis in particle accelerators, too. When two particles collide, some of their kinetic energy is converted to mass. As a result, we get many subatomic particles like electrons, positrons, muons etc.
To know how this process works in detail, you'll have to dive deep into modern physics (Quantum physics etc). One thing I'd like to say that its not a big thing like most of society think. 
