Choosing a Topic
This actually wasn't hard because I wanted to pick something that I have particular knowledge of. As a part-trained Oracle DBA with experience of Oracle and PHP, these two topics seemed like an obvious choice.
Preparing the 'Cast
I was quite determined not to read aloud from a script, so I put together some bullet points and practised doing the talk from those quite a lot. Playing back these attempts helped me to see what worked and what didn't - those points got added, removed and re-ordered quite a few times. Even after a lot of practise, I was still pausing too much between points. I ended up transcribing my unscripted version and then re-recording it, which worked well as I had my own natural language but no pauses!
Sound and Software
I was concerned that I would need additional equipment, such as a better microphone. However my partner has a macbook and the built-in microphone on that proved to be up to the job. I used Garageband from Apple's ilife to record the track (and also to edit out the clicking I got from pressing the mouse to start/stop recording!). This saves files in .m4a format, but this can be converted to .mp3 using itunes.