First you'll need to learn a programming language. N900 comes with a bunch of languages installed (c, c++, python, perl, and prob a few others) For a powerful language which is not difficult to learn, take a look at Python.

If you want to do User Interface programming, things get a little more difficult. You'll also need to learn a GUI library (either GTK which is currently the official, or Qt which will be the official library going forward) Since both of these libraries are meant to be used by C & C++ languages respectively, you'll need to use a third component which is called a Python binding. This binding allows Python to use the libraries easily. For the GTK libraries, take a look at PyGTK. For the Qt libraries, look at PyQt and PySide.

Research all of these and find out what makes the most sense for you. The latest version of Qt (4.6) isn't quite ready for Maemo yet (in testing, planned for release by end of 1st quarter) but I think it's worth waiting for as it will be the standard library for Maemo going forward, is very well designed, and can be used on other devices other than the N900.