I feel I must speak up and voice my humble opinion here.
If you want to learn to program, you should start as you mean to go on and make the effort to learn a language such as C++ or Java.
Yes you could learn Blitz Basic for quick results but you'll then be restricted to a single platform and stuck in a procedural mind set.
In practice, programming is easy, it's design thats hard which is why I feel it's vital to learn good OO design practices and a OO language such as C++ or Java.
Another benefit of these languages is the vast amount of resources freely available online from training sites to source code.
I think you will find that C, C++ and Java isn't really that difficult to learn.
All of these languages use a small command set with a small number of rules and constructs which can be combined in a unlimited number of ways, hence the power and flexibility.
To be honest, I'm a bit out of touch on the Amiga programming side, working as a software engineer and architect on the Windows platform so I can't really voice a opinion on the best software development toolkits to use but I'd have to agree with many of the posts above and say the only way is the C, C++ and Java way to get some really useful programming and design experience.
Incidentally, having not used a Amiga for about 10 years and having just picked up a new A4000 (yes, new and still boxed...I can't believe my luck :-), I would be very interested in knowing the state of play in the Amiga community for good development IDE's, compilers and toolkits.
What C, C++ and Java development systems are people using? Specialised development systems or GCC? Any guides listing the Amiga API set and guidelines?