For my MA thesis I am trying to conduct a content analysis based on social media posts. I have never done it therefore I would really appreciate any help to understand basis and possibly more of said method.
Krippendorff's book is well over 400 pages long. As an introduction, I would recommend Weber's little green book from Sage, "Basic Content Analysis.
Note also that content analysis ranges from more qualitative and interpretive approaches to more quantitative approaches that involve counting codes. Both Krippendorff and Weber, are on the more quantitative side -- which is where most media studies are as well.
I deeply agree with David L. Morgan. An interesting book on qualitative content analysis is: Schreier, M. (2012). Qualitative content analysis in practice. Sage Publications
I have not read Schreier's book, but I am generally familiar with her work, which I would list as toward the qualitative/interpretive end of the content analysis continuum.