Understanding the Big Bang is sometimes difficult for people.
It helps if you understand a bit more about the state of the universe just after the big bang. People just asume that all the stars, planets, rocks and gas in the universe were all just one tiny lump smaller than a pea. this is not true as cosmically nothing existed until quite a while after the big bang. what actually exploded was energy (protons, Nuetrons, electrons and lots of other small things) think of it like a spark!
As the universe expanded there was enough room for these 'Particles' to start interacting with each other. they soon formed Atoms starting with Hydrogen. (One Proton, One Nuetron, and one Electron) which is an explosive gas! and the universe was VERY hot at that time. After even more time gravity started to affect things grouping these atoms into clusters what we now know as stars! and the life cycle of a star and it going supernova leads to the creation of more complex atoms.
The only really difficult thing to understand is what did the Big Bang explode into?
Which is the same question as, What does the universe expand into?
Theres some REALLy interesting theorys on that one i can tell you.