I would suggest you do the following:

1. Install python on your computer and go through a python tutorial. It is a good language for learning programming, even though it is a bit loosy-goosy compared to Java or C++ and lets you get away with lots of things the other languages do not let you do. And though it is simpler to work with than C++, you can create first-class programs for the N900 with python (and the end user won't even know the difference).

2. Use python to write the meat of your program, minus the user interface. For example, if you are writing a program to convert among feet, miles, inches, and meters, write the python routines to do those conversions and get it working in a generic fashion so that when you later add the graphical interface, all you have to do is plug in the numbers. (In python you don't have to use Objects, which you will learn about, but to mesh with QT it will be important to understand how to program with objects.)

3. Once you have a grasp on object oriented programming with Python, you can install QT and PyQT on your computer and go through the tutorialsexercises for those. (Some might tell you to use PySide instead of PyQT. I don't have any dog in that fight...I just know that PyQT was pretty easy to install on my linux system and it works on the N900, and you can always switch later with little pain if you need to.)

4. Once you have a grasp on PyQT, you are ready to add a UI to your app. Get the interface working on your computer, then you can copy the app to your phone and test it there.

Step 1 should keep you busy enough for a while, and then you will soon be on your way to writing phone apps. And learning python is not just N900 focused...you will be able to use those skills for programming on any platform, or parley what you have learned into learning the specifics of Java, PHP, C++, C#, or any other language you decide to take up in the future.

Good luck.