With the increase in the time spent at home, social media platforms' role has become an integral part of the public discussion in the COVID-19 period. Individuals use social media platforms to express their emotions, interact, and engage in public debate. Therefore, it is essential to analyze social media platforms for those wanting to understand public opinion during the pandemic. This thesis is the first study that examines the Turkish Twitter-sphere to understand the change in public opinion during the COVID-19 outbreak. For that purpose, starting from 12 February 2020 (one month before the first announced coronavirus cases in Turkey), 4.3 million Turkish tweets with a broad range of keywords are collected until June 2020 to investigate the public opinion change on different topics and to examine the actors leading to that change. The scope of the analysis is not only health-related discussion but also includes a broader range of themes such as politics, economy, and disinformation. This study also collects 4.15 million Turkish tweets with keywords of vaccine ("aşı" in Turkish) from 4 April 2020 until 17 March 2021 to unpack the health of the information ecosystem. Preliminary results suggest that (i) religion is the prominent phenomenon in Turkish people's perception of the pandemic, (ii) and the Turkish Twitter-sphere is highly vulnerable to mis/disinformation operations, and (iii) several communities with divergent interests exist in the vaccine network.