M.W.
I'm going through the same thing right now.My daughter is 20 mo. and sees her brother going potty in the big potty and doesnt understand why she has to sit.we got her a childs potty and set it in the livingroom so she could get used to it(or wherever she plays most).after her nap yesterday i took her diaper off and sat her on it and she peed!!then i turned on a movie for her and sat her on the potty about every 15 minutes and when she looked like she needed to go and she did it again.you have to make sure shes ready though.there are a lot of websites to go to like http://www.drgreene.com/54_19.html . just be patient, positive and every time she goes in the potty get excited.also what helped my son was making a potty chart and everytime they go in the potty they get to put a sticker on the chart.hope this helps.good luck!
M.