Looking for a place to stay in July and August for my erasmus internship (close to old market square)

Hello, I am from Turkey,İstanbul. I will come to Poznan between June 25 - August 31 (2 Months) for my erasmus internship. I am looking for a clean, safe and affordable place to stay during my time in Poznan. I dont smoke, I dont use alcohol. I am a very calm and friendly person. My internship office is around Old Market Square, so I am looking for a place near that area or close to that area. I need to book a room urgently because I should apply to Shengen visa with my accommodation reservation. Thanks in advance :)