The easiest way to start at present is by downloading the Nokia Qt SDK. On Linux, it comes with a symbian and Maemo toolchain and device simulator, tailored for Qt development. AFAIK, the Windows version provides the same feature set for Maemo, and more features for Symbian.

If you want to do other non-Qt develoment, the Maemo SDK might more feature complete.

I would recommend you start with Nokia Qt SDK on Windows for now.