Thank to Blink i was looking for that too

joh, There are two ways to make a girl sit. You can refer to book "The ultimate guide to fl*rt" (or something similar) for further information.

But if you just want to just make a model of a girl (not a real girl) sit, you can use a modeling app like MED. you should move its (not her) bones or if it hasnt bones move it vertex to achieve a sitting position. it pretty simple and will not take more that 20 min. Try it.