I bought an excellent book called 'Lightroom - Classroom In A Book'. It has a series of lessons to work your way through and when I had completed the tasks I was an expert. Well, not really, but I knew enough to get me going.
Lightroom now has an importer from iPhoto and Aperture. I didn't use them as I preferred to start afresh, but apparently they work well.
After a few months, I now prefer Lightroom to Aperture.