Well, hmm. How big of a picture are we talking here?

For me, the one thing that has been my best guide for developing on the N900 has to be the Qt SDK. As an SDK, it isn't perfect, and has a lot more rough edges than you'll find in commercial SDKs, but man is it just chock full of useful stuff. Documentation, example code, the works.

(I should note that Qt 4.7 was the last version that had explicit support for Maemo. I think Qt 4.8 can still build Maemo executables, but you'll probably want to stay away from Qt 5.0 and greater...)

Another enormous advantage for developing on the N900 -- there's a huge library of high-quality open-source code available. You can learn a lot by just grabbing an app that you like, opening up its source code, and working your way through it.

I should say, if you don't already have a background in writing code, it might be helpful to first work your way through an "introduction to programming" book or website. Having at least a general idea of what a line of code can and cannot do can be a great help in understanding what folks are doing in their code, and why...