If you're still reading the comments: I think your intentions are good, but your approach might not give you the result you're after.
When my siblings were in high school and I lived far away, I sent them a copy of Changing Bodies, Changing Lives, which was the teenage version of OBOS, and the best book on the market at the time. I heard later that they all read it -- one of them openly, and two of them secretly. I didn't offer to discuss it with them, but I felt sending the book also sent the message that I would talk to them if they wanted to talk. [I didn't even find out that they'd read the book until my sisters were in their twenties]
And I think that's enough. I would give a book to her (wrapped or in an envelope) and just say "I read a book like this when I was your age, and I found it helpful."
A young adult librarian can help you figure out what's the best book on the market today, or you can look through them in the library and see which ones you like best. Most of them will have web sites listed in the book.
Your niece is growing up in an environment where there is a lot more information available about sex and sexuality, but good quality information is always helpful.
You want to help her experience be better than yours, which is admirable. You can't assume your needs are hers, though. She could be lesbian, or asexual, or not ready to think about sex just yet. Letting her control how she uses the information and who she asks for help is a way of respecting her -- and having good healthy self respect will help her in the wide world of dating and sex as she grows into it.
Buy the book. Say one sentence about it. Or say two -- "I remember being confused about a lot of things when I was your age. You know you can always talk to your Mom or me if you need help figuring stuff out." Don't wait for a response.
Then leave the subject strictly alone unless she brings it up. Teens want to think they know things already, and they want to figure things out themselves.
Other good web sites:
Internet Public Library: