First of all, unless your 250 pages are single, rather than double-spaced, this is not a novel. At best, it is a novella. But number of pages is essentially meaningless. What matters is word count (the number of words). Whatever word processing program you are using should have a word-count function. A novel is anything from 80k and up.
You are asking “how do you publish and sell” your work. I assume by this, you mean to self-publish. There are several ways of self-publishing, but the most well-known is probably through Amazon. Here is Amazon’s link to give you all the information you need about doing just that. Amazon.com : Take Control with Self Publishing.
As for how you sell your fiction, if you are self-publishing, you are going to have to research self-promotion. Everything will be in your hands, for good and ill. You will have to invest a lot of your own time in learning effective self-promotion and putting what you learn into action. There are lots of internet sources to help you to learn how to do just that, but after that it will all be up to you.