Potty training my 2 1/2 year old

Hi I was wondering if you could give me some advice. I am trying to potty train my little girls it is not going to well. She will hold her pee rather than go on the potty. She was holding herself this morning and I kept on putting her on the potty. She was getting very distressed so I put a nappy on her and she went to the toilet. I am not sure where to go from here!
thanks in advance