I say wait until you think they are definately ready because it wil make ur life so much more difficult! I think my daughter was actually ready before we started but I waited until I had some time off work to get things started.
We basically had a potty available for her at all times and tried to let her run around with nothing on her bottom half and encouraged her to go on the potty if she needed to go. We put her on when we could tell she needed to go, just after food, drinks etc. She got it after 3 days and we have never looked back, she ha the occasional accident but only usually when she is so busy playing, usually no 2 with her! She is not dry at night yet but it won't be long.
There will be accidents and you will know if she is ready because they usually pick it up quite quickly.
Lots of praise when she does something on potty and just ignore if she has an accident, it's not their fault and she will get it soon. How old is ur daughter?
Laura x