Best place to start is Google.
HTML is relatively easy (which doesn't mean it's easy, just easier), but expect your first very simple application to take you a pretty long time.
I like this site personally: HTML Tutorial
Its going to take you too long to do it that way. Assuming you have some dev background, i'd download the free sdk and download the free demo apps and learn it by walking through the debugger.
Personally, to make a game for playbook (I currently am making one) I use Game Maker HTML5. It is $100 but it is simple to use and you can make kick*** games using it. Apps are a bit different. For that, you need to use manual coding.
However, for apps I use the android app maker and then convert to playbook. Works wonders.
It is fairly straight forward, so you should be able to pick it up pretty quick. And if you get stuck, at least on the BlackBerry end of things (compiling, signing, etc) I'm sure you can get help here in the forums.
Personally I use notepad++ and handcode. It's free and it works. But if you're going to be making anything really intense, or games, you might want to try a air or the native sdk.