The paper I want to publish is just a conjecture.
There are several ways in which conjectures propagate, and each has its own method for publishing.
If your paper does not have any results apart from the conjecture, then such are usually disseminated informally. Tell your friends, colleagues, publish a blog post, ask for a proof on MO. This is a great way to find someone to work with in helping you turn your conjecture into a theorem.
If your conjecture is supported by partial result, then as pointed out by Artem Kaznatcheev, a journal suited to this results is the good choice.
If it is supported by numerical computation, you can aim at "Experimental Mathematics".
If it is not supported, or supported only by previous results, then it would probably be difficult to publish. Sometimes conference proceedings can be a suitable place, but informal dissemination would probably be the most common practice.