Make discussions age appropriate. Scale up the detail as they get older. NEVER LIE TO THEM. Just leave out details.
My kids were all born about 3 years apart. This left ample time for each of the previous children to ask questions typical of their age group. Answering their question on the same level is always best.
An almost-3-year-old asking, "how does the baby get out of your tummy?" probably doesn't warrant details on the labors and pain of ordeal. Rather, "a doctor helps get the baby out," will suffice a typical 3-year-old.
My 6-year-old daughter was keen enough to ask how the baby gets in there in the first place. This was answered by a basic discussion of the mechanics of sex, followed by, "you have to be this tall and married." However, she has always been fascinated with biology, and wants (and probably will become) a veterinarian. So, she already had investigated things on her own accord, like development stages of the baby, knew that "moms have eggs", and asked "how many eggs do you have?" (We consulted Google -- about 3 million eggs, but only about 300 become active.) So, we answered her questions as they arose. She understood these things, but not how the whole process was initiated.
At 10-11 years old, a child should be fully aware of the maturation process, because, "weird things start happening to your body." They need to know that it's normal, and why it is happening.
Using proper names and terminology is important, too. You may need to look up the names of things (I assume you're not a physician), and/or look up some diagrams to review with them, if you deem it appropriate. After all, a picture is worth 1,000 words.
Springing the entire subject on them all at once is not a good idea. Initiating a discussion that immediately delves into the subject will make them feel awkward and/or embarrassed. Talking through the facts openly, without showing embarrassment, is important; because it shows them that sex is not a bad thing, and is a natural part of life. They will feel much more comfortable about asking their own questions, too. Setting expectations about sex is also important. Let them know your opinion of an appropriate time for them to become sexually active (no sex before marriage, etc.). Discussion your opinions of birth control should also be held.
If you feel you need to have a discussion with your child, you may try gently steering a private conversation to the subject. If they don't want to talk about it just then, don't force the issue. Do, however, attempt to revisit the topic that evening or the next day. It's important to eventually talk. Ask your share of questions, too.
With knowledge comes responsibility. Don't let them become irresponsible through ignorance.
You'll do a great job!