The head determines the class.
For example:
Dragon elf head = Mage class, depending on the quality of the head, your creature will know Magic Missiles rank x and another spell (magic blast?) at rank x where x determines the head's quality.
As far as the other parts go, you should be able to click the buttons next to each part (head, arms, body and legs) and different parts will be used. You can check out your creature's stats at the bottom of the page, making it easier to decide what parts to use.
I hope this helped,
- Merdarion