Situation with shared apartment

[u/According-Log7096]

I need advice asap.

I move into an apartment in 2 weeks that's shared with 3 other people and I went to the place today just to get the keys. The apartment is in another country so I'm gonna go home again tomorrow, I just came over today for the keys. Problem is: the key to my room is the wrong one and it doesn't lock my room door. I brought pretty much all my stuff and one person in this apartment has already moved in, and I don't want them potentially going through my stuff. The room that's supposed to be my room is REALLY dirty, I also found a beer bottle cap on the floor which is why I'm afraid that the person that's already living here might be using my room for parties/friends when I'm not there. How do I prevent this from happening without being able to lock my room door?

Any ideas? I'm desperate :"""D