You asked

When should I begin preparing my daughter for getting her first period?

It’s important that you discuss menstruation with your daughter as early as possible as some girls can get their periods as young as eight or nine. Also, many girls in her class will have the facts, some of which will be wrong. It’s important to set the record straight and offer her some basic advise to set her mind at ease.
Sit her down one day when it is just the two of you and you can be relaxed and tell her about the facts of menstruation. Tell her it’s all very normal and it happens to every girl. It can be a good idea to also give her some reading material that she can look at when she is on her own. 
Make sure that she understands that she can come to you at any time with any questions or concerns she may have. 

More questions

Children can learn invaluable lessons from winning and losing. 
Dealing with children who are dishonest can be one of the most difficult behaviour problems that a parent has to deal with. However, it is important to realise that even though their behaviour is wrong it...
When you tell your child that it’s time for bed and all you hear back is, “You’re stupid!” Does this mean that you have years ahead of you of being back-chatted?
Discussing the birds and the bees with your child can be a difficult subject to broach but it's important to get it right. 
It's important that you discuss menstruation with your daughter as early as possible as many girls get their periods at quite a young age.
It's important that your child knows how important it is to always tell the truth.
Children can learn how to tell lies from a very early age, commonly by around three years of age.
School children lie for many reasons depending on the situation and the reason for lying
How you react is important when trying to get your child to stop using a particular swear word.
Swearing is a common behaviour issue in young school-children.