This is one area where you should pay particular attention. I've read review after review where readers write that they didn't connect with the characters in the story, and/or the characters felt one dimensional. As a writer, you must get your readers connected to your characters enough to want to walk alongside them in your story, or at the very least, to watch from the sidelines as your characters go through whatever you've beset them with. Without this connection, your reader never builds a vested interest in your characters success or survival. That's bad for reviews, and bad for sales.
Having said all that, the worst way to introduce your character(s) in to dump everything you know about them into one paragraph right out of the chute. This is not necessary in any circumstance, and should never be done. When you write, you should be ever mindful of layering your story. This helps build intensity. Spread your descriptions out over a few pages if you insist that the reader know every detail about your character.
If you're inclined to see excellent examples, I'd highly recommend two books I've read recently. Both very different, but prime examples. The Grid by Carlton Winnfield is narrated by the shadowy operative employed by The Grid. I don't recall one single mention of this man's looks, height, weight, etc. Not even the color of his eyes. But, through his writing, what I did get was that the operative was very agile, highly intelligent, well trained, and extremely dangerous. And that was precisely all I needed to know about him. The very nature of the story demanded this vagueness, yet I did feel like I knew him on some level. I connected with him.
In complete contrast, you have Partly Sunny by Terry Lee. Darcy, our lead character here is very materialistic, and her main concern in life- in the beginning, is designer clothes, shoes, and living in the "right" part of town. As such, she is described in great detail, as is everything in her life. It's an incredibly realistic look at one young woman's life, and how she comes to realize what's really important, but here again, this story demanded such details be told. The author, however, spread these descriptions out whenever possible, and it just works. I never disconnected from the story because of overwhelming details- despite the fact that there were a good deal of details given. It's all in the way you handle the writing, and the author did it perfectly.
Knowing when enough is enough is key to building your character up for your readers. Don't be afraid to leave some things to the imagination of your readers. Take into account the genre of your book, it will often dictate just how much information you need to impart. A romance novel, for instance, you'll typically find a fair amount of character detail, whereas in a detective, or spy novel, you may find only simple details.
When you start a new novel, only you can determine how much will be enough, but in the end, it's your readers that will judge whether you went to far too soon. Find that happy medium and you will have mastered one of the hardest parts of writing, and one that causes the most negative reviews.