Seems like some good info in here already, but I'll chime in as well.
Twirling your hand by the spotter/driver can also mean "hey we've gotta do a U-turn" or something similar.
When backing a trailer up, steer from the BOTTOM of the steering wheel, big help.
Bigger people = more speed to 'pop' them up on skis/wakeboard. Me/my dad require a WOT for about 5-10 seconds, and during that time we're pretty much under water, but pushing it out of our way, then we pop up and cruise just fine at 30ish.
For wakeboarding it doesn't take much for someone to get up since its a bigger surface, but you still need extra power at the start.
Commonly used words are "HIT IT" from the person in the water to let you know they are ready to go.
Before taking off make sure you have the wheel straight and the rope is tight/no slack.
When turning with people in tow, or in general the boat slows quite a lot depending on the degree of turn, so always add power when turning, esp if people are standing, they'll sink.
have fun man, I've been boating for the last 18 years, awesome shit, check out lake cumberland one summer if you think about it, rent a houseboat with friends, I garuntee its the best damn time ever!
Edit, almost forgot, you might want to just watch your RPM's as they are more accurate than your speedo. I know I tow my dad at 32-3400rpms, which I think is mid 30's, but like I said, I watch the rpm's esp when starting because they tell me where I'm at, and sometimes the speedo thing has to adjust, etc.