Is it possible to publish a paper on a topic about which you already wrote an online article? I am a student and so I have no experience in publishing papers and I do not know much about it. My question: Is it necessary for publishing a paper in a journal, that you never have published something similar on your homepage / blog or on another online project?

I have the feeling that this is not a problem, because the own written online article on the personal blog was never peer-reviewed and publishing papers based on a dissertation does not seem to be a problem. Am I right?
This is perfectly acceptable.  It is, in fact, fairly common to "upgrade" a white-paper, technical note, or other such non-peer-reviewed publication into a peer-reviewed publication.  Two important caveats, however:


The appropriate form for a peer-reviewed publication is generally very different than for a blog post or similar informal format.  You will likely need to almost entirely rewrite the publication in any case.
If the other material is durably available or has been circulated in academic circles previously, however, you should acknowledge this fact and reference the prior publications.  This may be in the form of a citation, or in the cover letter of the article, or both.