I want to make a purse cake do I need a pattern and what pan do I use
It all depends what size cake you would like to make. I usually bake 2 9x13x2 cakes and stack and carve from there.
It is not necessary to have a pattern, but it may help if you are not confident with your sculpting skills. Just cut the basic shape of your cake out of paper and place in on the cake to score an outline.
You may also want to look up cupcake envy on youtube, they have a video on making a mini purse cake, and the instruction would be the same for a larger size.
I agree with calivettie's method. Don't forget to use supports if the cake is more than 3 layers tall. Another good resource is the Confetti Cakes book, your library may have it. They give clear instructions and pictures.