Re: whistle
761349,
I also will say that you should just keep going through the same lesson for a while. If you practice diligently, while neither straining nor stressing for 6 out of 7 days a week (I think you should get a day of rest each week, personally) for at least 2 weeks so that the excercises become rather easy and relaxed, then you can move on to the next lesson.
Also, I agree with musiciano (though I don't think he's posted here yet) that you shouldn't really care very much how good you sound as you're practicing. When I was doing the nays at first, I wouldn't put enough of a "cry" on it, and I ended up straining and going into falsetto.
So, basically, don't get "stuck" in one lesson, but make sure you practice diligently and for enough time so that your voice and you are ready for the next lesson.
Oh, more posts have been adding as I was writing this and going about other things...
As far as hitting the top note (usually it's C above women's high C), I haven't gotten that high yet, and I've practiced with it for some time, so I'd say to just do what's comfortable. Your voice will grow and mature, becoming looser and more agile (or whatever it does). The point is, your voice will improve if you practice with whatever you've got if you're practicing in a good way.
And, keep the wonderful determination!
...Just don't get ahead of yourself...