We just returned from our first visit to Typhoon Lagoon and I will echo the above response. They opened the main entrance/turnstiles around 9:35 or so and let people back to the initial shops/locker area. That's a good opportunity to get a locker or get in line where they hold the crowds. I was a little surprised at the mad dash to get prime shady spots once they let people past the ropes at 10:00. People running all over the place.
I HIGHLY recommend, if you can swing it, reserving a Getaway Glen umbrella. I had called the day before to see if any were available and I was told there were none available. However, once we were let into the initial shops/locker areas, I noticed they had a sign outside Singapore Sal's advertising umbrella and cabana rentals. On a whim I went in and asked and they had several available. The cast member told me that sometimes they wont allow phone sign-ups within 24 hours. The whole process took less than 5 minutes and we went back out to get a locker and get in the line. We picked Umbrella 27 and it was just about perfect (I don't recommend the umbrellas in the actual Getaway Glen section over by Leaning Palms as we went by there later and it looked very crowded/cramped with many umbrellas in a relatively small roped-off section). 27 and 28 are right next to Crush N Gusher, the pool at Crush n Gusher and Miss Adventure falls as well as a bathroom. And there are only two umbrellas in that space with tons of room and it is set off very privately away from other guest areas. It was well worth the $60 just for the great space alone, not to mention reducing the stress of the mad dash and trying to lock down a shady spot - especially for us as it was our first time and we weren't even sure where the best spots were anyway.
Even with renting an umbrella, I would recommend getting there at rope drop. We were able to do all of the slides, including two laps on Crush n Gusher before noon so we could enjoy the wave pool and lazy river after lunch. And this was during a Spring Break week, too.