It may seem like a basic instruction, but before you seek any kind of representation or publication, make sure the first draft of your novel is complete. You would be surprised by number of writers, looking to get their book published, seem to think that it is possible for them to secure the representation of an agent, or get a publishing deal, even if they have only completed some of the chapters and the synopsis.
I know this is the bane of many publishers because, if they like what they read in a submission sent to them, they want to go on to read the rest of the book to ensure that the quality and energy in the opening is sustained throughout the novel. Theoretically it might be possible to get agreement to take your book to the next level without the novel being complete, but it is unusual, particularly if the work is by a debut author. Any agent or publisher, even if they love your idea and your sample chapters, will require to see the whole manuscript.
Put simply, the agents and publishers want evidence that your quality of writing is sustained throughout the novel. Writing a novel is not an easy thing to do and therefore it is important that they see you can properly plot and pace your book. Although this is important, it is not the only thing they are looking for. You also need to prove to them that the character development and other aspects of writing a long piece of prose are within your capabilities.
The agents and publishers must also know that you have the energy, commitment, and resilience to finish a novel. If you are not a professional full time writer, this last point may be the most crucial. There are few writers who have not written at least some chapters of a novel they have never finished and if your writing has to compete with running a home, an external job or organizing a family, setting aside time to complete a book can be challenging. To write a novel is an enormous commitment. No agent or publisher wants to make an offer unless they are sure that you will complete the writing required of you within the time limits they set.