I think you should wait till yor child shows an interest in going my daughter was out of nappies by the age of 2(2 and half ish at night time) mainly because I worked at a playgroup and she came with me , she watched the other children use the potty and she wanted to try it.
I also think(if child is ready) that if your going to do it, just do it, buy loads of cheap pants and not these pull up things as they dont know when hey have weed and they feel; too much like a nappy. There will be accident but they will soon feel how uncomfy it is.