I agree with Cherie, research it online. The process, if you want to get a publishing company to foot the bill and market your book is very daunting.
The other thought is to self-publish and offer your book to friends and have a few copies on Amazon or Barnes and Noble, that you pay for yourself and hope to sell.
Have you had a few trusted readers read your book and offer their opinions? Honestly, that's the first place to start - with your school librarian, or English teacher, or other adults who might like to read Science Fiction giving you a critique.