If you haven't seen Mamma MIa, it's terrific. I've seen it four times (Minneapolis, London and twice in NYC) and would go again. The Lion King is awesomely staged, and I'm fascinated by Gray Gardens, but you should read about it before you go.
Most Broadway shows have websites that give you plot outlines, etc. Check them out as well.
Enjoy it, but if you have time, make sure you see the theatre exhibit at the Museum of the City of New York up on 103rd and 5th Avenue, in Spanish Harlem. It's fantastic! It's on the third floor and has great stuff from Broadway's past. Last time I was there, they had the Gypsy Robe on display. It's a little museum that no one knows about, but for theatregoers, it's a MUST!
If they have the silver tray that the Theatre Guild gave George Gershwin on the opening night of Porgy and Bess on display, you've struck theatre gold! They also have Mary Martin's nun outfit and sailor costume, one of Yul Brynner's King outfits and one of Lansbury's dresses from Sweeney Todd!
NYC Visitor and Broadway Fan