I was a PhD student and working on a paper. My adviser told me that he could not support me anymore so I left him. Now I am wondering if I could publish that paper on my own. There were 3 other coauthors on that paper but I was the first author and main idea was from me.
Your paper had other authors. Generally, if you want to publish it, you have to communicate with the other co-authors (whether they were your supervisor or not).
Regardless of who the first author on a paper is, if other individuals made authorship-worthy contributions to the work, you can't publish it without communicating with them first.
When an academic paper is produced, the copyright generally belongs primarily to the institution at which the work was done and secondarily to the author(s) of the work.
Therefore you must seek and obtain the permission of the institution at which you were enrolled before disseminating the work in any form, whether electronic or otherwise or presenting it at a meeting or conference. Failure to do this would represent a breach of academic ethics and could leave you open to legal proceedings from the institution in question.