This would be a pretty good start:
http://pregnancy.about.com/cs/secondpregnancy/a/babiescom...
Updated**You can also poke around the SIECUS (sexuality information and education council of the United States) website for lots more information regarding comprehensive sexuality education. You probably don't need a lot of the information now, but it can definitely be helpful as your child grows older. www.siecus.org (also, sexedlibrary.org is a great resource area)
Start really basic with her - you don't have to launch into a clinical discussion regarding conception. Above all, however, be honest with her. Please don't tell her that a seed gets planted into mommy, and avoid telling her that the baby grows in the belly (do you want her to approach men with beer guts to ask them if they are pregnant?). Personally, I prefer using appropriate terminology when it comes to body parts instead of silly made-up names. (Of course if 'uterus' is too much for her right now, just say that girls have a special place that can help babies grow...when they get to a certain age)
Believe it or not, the most important aspect of this discussion will be how you handle it. If you are calm and patient with her, she learns that YOU are a TRUSTWORTHY source of information when it comes to sexuality, that discussing topics of this nature are normal and natural. If you are hesitant, fidgety, and try to shut her down, then she learns that sexuality is a shameful topic not to be discussed. Providing the right type of nurturing environment will pay off greatly in the future, as you are seen as her source of accurate information regarding sexuality and she'll be more likely to follow your family values.
Good luck to you - fortunately my little guy is only 10 weeks old and it will be quite some time before we have to have this conversation!