How do I modifiy a ready made character - Andrew In Bedford, UK

I have recently started using blender and after seeing some of the excelent animations online and would like to have a go at creating anything even almost as good.

I have some models of people and would like to know how to change their shapes. Once I know hose to do that I will have a better idea of how to create my own as I will see how the frames work!

Thank you in advance, 

One more question and this is more of a plea than a question, I recently found an animation called snake lake and am wondering who created it and what software they used to make it. So one off chance that person is a member here maybe he or she can tell us all! Thank you again!