I want to start making up my own music (trance/pop/rnb) using Reason. Apart from my PC and USB MIDI keyboard, what else do I need?

Reason is great program, but is very limited without what Propellerheads calls "refills" or "soundbanks". A sound bank is basically a large collection of samples, pre-configured for use with Reason. A refill is generally just a smaller version of a soundbank. Reason comes pre-packaged with 2 large sound banks, named the "Orkester", and the "Factory" Sound Banks. Reason does have the ability to synthesize electronic sounds without the need for samples, although the sounds produced are relatively uninteresting, and result in very one-dimensional tones. Once you feel the need to explore new sample collections, you can check out the official Propellerheads refill page and download quite a few refill packs for free:

http://www.propellerheads.se/download/refills/