You must study the past to understand the future.
Whether you're doing a lot of "soft science" hand-waving, or you're obsessed with consistent "hard science" numbers, you need to know what you're doing. You need to have a plan in mind for what lies ahead. A good way to begin is to look backward. Where did you come from? How? Why?
These are incredibly daunting questions. Whether you hold multiple degrees in biophysics, engineering, and predictive political science, or if you're just a kid with a keyboard and a big imagination, you can't possible keep everything straight in your head. That's where I find this site very helpful.
http://www.historyoftheuniverse.com/ and detailed timeline.
I know it covers a lot of ground, so take it a bit at a time. Read one chapter every few days and mull it over between readings. I promise, you won't get bored.
Let's say you don't want to build in the aspect of creationism versus evolution. Fine, skip that. If it's not essential to the story you want to tell, don't stress about it. Maybe the clash of science versus religion is your bread and butter, the cornerstone of what drives your tale. Or maybe science and religion live in complete harmony. Either way, you need to know what you're talking about. Study up.
But if you're not so concerned with what came before and you just want a basic breakdown of where we are now, well then, the last few chapters are enough to get you going.