Honestly, I think you should just hand her the Lord of the Rings and tell her to read it. There's no way to really describe it.
Or if she's not a massive reader (and to be fair, LotR is a massive book), why not make a shared project out of it. Listen to the BBC adaptation or an audiobook version together, or maybe you could read the book to her as bedtime-stories.
If this still seems too much, read her your favourite poems, show your favourite pieces from Tolkien illustrators and if you fancy the PJ or the Bakshi movies, watch them with her. Most importantly, don't flood her with information and names of people and places, and stick to LotR first. If she reads it and gets enthusiastic, then you can start talking about the Silmarillion, but mind you there are many people who love LotR but don't really care about Sil.
All in all, if she's already curious about it, it shouldn't be too difficult. Don't stress about it!
