How do I get a kitten to use the litter box?

I just got a new kitten. She's a bobtail and very sweet. However she won't use the litter box. She can hold it because when I went out I put her in the large garden tub where she could not climb out. I put the box in there and food, water and a blanket. Two hours later I came back. I took her out of the tub and the moment I did she ran off and quickly went to the bathroom on the floor. What can I do about this?