Thanks everybody, who read my post. I found a solution myself in the article 'Alfresco Developer: Working with custom content types' by Jeff Potts.
So, now I can create my custom contents. But problem ist, when I add new content, and give it the type 'whitepaper' (like in the above mentioned example) it does not add the desired aspects by itself. What I need to do is add a content rule to the space with the desired aspects. Then it works.
Is this the way it should work?
Can anybody help on this?
Thanks a lot.