The issue of plagiarism is mainly concerned with misrepresenting the nature or source of the written work that you are publishing.
The most well-known, "archetypical" type of plagiarism is using someone else's ideas in your writing and not citing the source, which may mislead readers into giving you the credit for these ideas.
Another type of plagiarism is using the same wording as your source, but not clearly indicating this via quotation marks or other quotation devices, which may mislead readers into giving you the credit for coming up with this wording.
The idea of "self-plagiarism" is more complicated, but it mainly refers to an academic concept. In many cases, when you submit a writing assignment for an academic course, or to an academic journal, there is an implication that you are submitting new work. If you re-use some of your own old work without making it clear that that's what you're doing, it might look like you're trying to mislead your instructor or publisher into thinking they're getting new work when they really aren't, and you may get in trouble for this.
The other thing that can be connected to the issue of "self-plagiarism" is copyright infringement, as Terri Simon noted. I can't say for sure without knowing the details, but I would be very surprised if you didn't retain all copyright to work that you did as a student. Generally copyright is only an issue if you signed over the rights to your older work to some publisher.
Publishing a novel or other work of fiction is not like publishing a paper in a journal, or submitting a writing assignment for a course. When you publish a work of fiction, there is no implication that you are presenting new ideas or ideas that you came up with yourself (many authors re-use the ideas of previous authors without giving any explicit attribution, or re-use ideas from their earlier work), so you shouldn't worry about this type of plagiarism. The wording is your own, so you're not misrepresenting someone else's wording as your own. I would think that you own copyright to the wording, so that shouldn't be an issue, as I said earlier. The only way I can see this being an issue is if the publisher for some reason finds it important for what they publish to have completely new wording, but I can't imagine why they would care. To make sure this isn't an issue, you should simply ask your publisher if what you're doing is OK. If you're self-publishing, that obviously wouldn't be an issue.