The reason creationism is hard to believe is because in our world, EVERYTHING has a beginning and an end. It's hard for us to grasp the concept that God always was because everything we see had a beginning and will eventually have an end. But if we think of it that way(reguardless if you're a christian and believe that God created the universe, or if you aren't and think the earth and planets were created from a huge explosion in space, or whatever other belief you have), that everything is created, you eventually have to say, everything was created by something else. And you can keep going back and saying "d" was created by "c" and "c" was created by "b", and "b" was created by "a" and eventually you get to something that always was.
It's really hard to explain this because it's hard to wrap our minds around it. I mean, if you look at a family tree... well I was created by my parents, who were created by their parents, and were created by their parents, etc etc. Well eventually it gets to the point of the first person that was. And that same principle can be used to argue this.
Eventually you trace everything back to 1 thing that always was. God said that he is the beginning and end. God always was.