To learn basic programming concepts I would recommend Python. It's easy to get started with and you don't have to care about the nitty gritty details. I can wholeheartedly recommend the free book Think Python, which teaches you programming from the ground up. After you have read through and understood the contents of that book, you will know all the basics of programming. The journey of learning doesn't end there though. It takes a long time to become a decent programmer, so you have to keep at it!

If you want to write apps for the N900 I would also recommend Python. Just keep in mind that performance is an issue on small devices. Personally, I wouldn't use anything but C++ for "real" N900 apps because of CPU and memory concerns.