Scientific poster about ongoing work I'm attending a summer school for graduate students, where I'm required to prepare a scientific poster about the research I'm conducting. However, the project is far from being finished, I only have some preliminary results.

How should I indicate on the poster that it's an ongoing work, and the presented results are not final yet?
How should I indicate on the poster that it's an ongoing work, and the
  presented results are not final yet?


As @buffy mentioned, this is certainly not an uncommon thing to do. I actually really enjoy presenting work in progress via poster, because, as opposed to already-published research, the poster viewers may be more inclined to give feedback on the methodology and/or research questions/hypotheses. 

It's not necessarily vital that you specify the research is ongoing on the poster, but if it makes you feel better you can say this is a pilot result or preliminary result. 

An aside, but important, I suggest first figuring out why you are presenting the poster--that is, what do you want to get out of this experience? For example, are you at all interested in method feedback? Do you hope to refine your research questions? Are you going for networking only? 

Cater your poster and your discussion topics to your needs while designing a poster which helps you achieve your aims while effectively conveying your ideas to the audience. 

Also an aside -- don't stress. Posters are pretty fun, especially if you know why you're there. 

Another aside -- if you know the conference attendees and there is anyone you want to meet or get feedback from, email them. Tell them you would love if they could visit your poster on Day X at time Y.