Mandolingirl…a couple more quick notes.
First I reread your message and saw that you have a Generation F that you find a little easier to blow. If that whistle sounds good to you, there’s no reason you can’t use it to learn on. It will be more piercing in the second octave than the D, but if it’s not squeaky, it will still be easier on your ears than a squeaky, leaky D.
A D is usually recommended for two reasons: 1) most traditional music is written in keys easily played on a D whistle and 2) because of that, sheet music and tutorials for the whistle assume you have a D. If you’re not playing with other musicians, the first isn’t a problem for you, and the second can be easily got around by thinking in terms of finger placement rather than pitch (easiest to do if you don’t have perfect pitch). In other words, assume that, when all your fingers are down, you’re playing a D note (even if you know intellectually that it’s a F), and finger the written notes accordingly. One of the nice things about whistles is that you can use the same fingerings to play songs in different keys simply by switching to a different whistle 
A few notes on “blowing” (I’m assuming here that you don’t play any other wind instrument), based on what I’ve seen other beginners do wrong:
First, you don’t blow the whistle so much as you breath into it. In other words, don’t blow each individual note…instead, take a good, deep breath from the diaphragm as you would if you were singing and release it slowly and gently into the whistle, playing an entire phrase (or two, if they’re short phrases) before taking another breath. As you get into playing more complicated tunes, you will want to be thinking about where in the tune you will want to place the breaths so as to interrupt the flow of the music as little as possible…again, just as you would if you were singing.
Second: There are several ways you can articulate the individual notes. The most commonly used by beginners is tonguing. You place your tongue up behind your teeth and whisper the syllable “too.” Try it without the whistle first…just sing a scale “too, too, too, too, too, too, too, too.” Now try it without singing…just whisper it. Next try it with the whistle, playing up the scale, whispering “too” for each note (be careful not to let your tongue touch the instrument). That’s tonguing.
In reality, you won’t want to tongue every note when you’re playing a tune. Most traditional music is very legato. A good rule of thumb is to tongue the first note of a tune, the first note after a rest, and the first note of a run. Otherwise, you only tongue if you want a more staccato sound (you can even make a note VERY staccato by whispering “toot” instead of “too,” using the tongue both to start and stop the note), or if a note seems to need the particular emphasis.
The other way to articulate a note is by using ornaments, particularly grace notes. I’ll leave it to Brother Steve’s site to explain these, and to your own ear (once you’ve done a lot of listening to good traditional music) to decide when it’s appropriate. Irish music in particular relies heavily on ornamentation, and the correct use is best learned by listening.
Getting a good sound in the second octave depends on breath control more than anything else. You need to breath more forcefully to get the higher notes, and how forcefully depends on the individual whistle. Some are very sensitive…they will squawk if you overblow and drop down to the lower octave if you underblow (my Feadog is like that, and my Susato is even worse in that respect). You may want to practice playing slow scales up into the second octave to get a feel for just how much breath it takes to get a good, clean note up there…after that, it’s a matter of practicing in order to get a smooth transition (note: this is something you’ll probably need to practice with every new whistle you get, as they really can vary widely in how easily they make that second-octave transition).
Finally, when practicing tunes, work hard on getting the notes and timing correct…don’t worry about speed (even if the tune’s supposed to be a fast one). If you go for speed too quickly, you will begin to make mistakes that can rapidly become habits. Better a reel played well at half speed than one played quickly with a lot of mistakes!
One final note: You may already know this, but just in case…don’t hold the mouthpiece in your teeth! (I mention it because I see the kids in recorder class biting their windways almost to a one). Hold it between your lips
Hope this is of some help.
Redwolf