Excellent question. Especially since these days I actually have an answer for that.
Lots of writers swear by those character profile forms you can fill out. I was very good at printing out those kinds of things, but every time I tried to fill them out, I got too bored to finish even one.
So I used to use my first draft as a way to get to know my characters. This led to lots of revisions since I didn't get the hang of my main character's voice until about halfway through. And for secondary characters, it took longer.
And then I read The Art of War for Writers by James Scott Bell (I know I talk about this book all the time. It's just that good.) He talks about a technique he uses where he journals for his characters. I've talked about it on here before. (Click here to read that post.)
I'm completely in love with character journaling. Sometimes it comes to me way easier than the actual novel writing does. And since I'm working on a new project right now (or, to be accurate, reworking a project that I love so much I'm never happy with the way it turns out), I'm actually doing character journals right now.
I always start with my main character. I ask them some kind of question ("How do you feel about your name?" "What's your relationship with your mom like?") and then they just take over. Usually for pages. It's particularly useful when you're trying to get in the head of the antagonist and other secondary characters. It gives them a voice of their own, motives of their own, etc. Remember, they have a story to tell too. The world doesn't revolve around your main character, regardless of what they may think.
All those other details - birthdays, a favorite color, appearance - I jot down on a spreadsheet as I write the first draft. I find those things don't really matter. Like, it's important to be consistent, but the reader cares more about your characters insides than outsides. Start there and the rest of that stuff will work itself out.
Have a writing question? E-mail me. I'm serious. Most of you apologize when you e-mail me with your questions, but I really love receiving them. Makes my life fun, and my blogging easy :)