I had worked on a project for my bachelor thesis in 2012 and in 2013, I moved to Germany for further study. Now when I am facing several interviews for Ph.D. and the supervisors ask if I have published my bachelor thesis or not. So I was thinking to add some computational data and publish my work in a good journal. But I noticed my supervisor with another student had already published my work in 2015 and I didn't know anything. Another student had a similar project(we worked together when we were working in the lab, we just had different solvents/medium for our samples) so he merged both projects together and published it and my supervisor was the first author. And anyone can find my thesis if they search in my university website with my name and the project name. Now my question is what steps I can take? It's my work and I worked for 1 year on this project and they didn't even mention my name anywhere or never asked me about this. Should I write to my supervisor regarding this? or should I inform the publication house? (I did not have any contact with my supervisor after I graduated)
There is probably little you can do formally or legally. Not citing your thesis was a lapse, but getting that changed is not likely to happen. You can, of course, point people directly to your thesis.
I don't see any ethical issues with the publication itself. The work was, in some sense, unfinished. It is natural to want to finish it and you weren't there to help. But you should have been cited.
But you might also be able to obtain a letter from your old supervisor explaining that the publication in question incorporated prior work of yours. That might be enough for your current purposes.
However, you might also be able to publish your thesis unchanged with original dates attached. It would be a late publication but if you can convince a journal to publish it, you could establish priority on at least some of the ideas. Your university will verify that the work was done prior to the new publication's date. If you change/update it, however, you lose the advantage of priority.
But try to work first with your old supervisor.
Another option is to extend not only your own old work but the publication itself. Then it is new work. You can cite your old thesis and the newer publication in this work. Ultimately that is what you want to be able to do, of course.