First of all, congrats on finishing on your novel! You have done something many people have tried but didn't complete! At this point, you have succeeded in something so pat yourself on the back and celebrate!
The job of being a writer is not yet finished, actually you've only just begun! Yes, being a professional writer also goes past the actual writing. Now you need to be looking for publishers and agents.
Always do your homework. You should already know the genre of your book. That is important. The same thing goes for the audience. They both determine who your publisher will be. You should know your story inside and out, and be passionate about it. Believe me, it shows through your emails and snail mail.
Many publishers are going email now so to save time now, always check their requirements before sending them anything. Most rejections come from people not paying attention. Its sad but more rejections are served than acceptance letters are sent out. Publishers and agents are looking for reasons to reject a story. Its not personal, its business. Don't give them any reasons to reject your story.
That is why editing is very important. I can't emphasize enough on that. If you're not ready for this phase, keep editing. Get more help if you need it but your story has to be perfect before sending anything out to anyone. Seriously.
Keep a positive attitude and be professional at all times. Never send out your whole manuscript unless told other wise by the publisher or agent.
Query letters should be professional too. This is the first thing the publisher or agent will see and will judge your story and your writing ability by this one letter so make sure its perfect. Check your spelling and grammar on this before sending it out either through email or snail mail. If you have any experience on writing (even if its minor), include it. Specify why you're the best writer for this story. Even personal experience (if it relates to the story) can be helpful at this point. And don't forget to include what the story is about. Keep it under 3 pages (1 is best).
I don't want to over whelm you so this is all I'm going to share for this session. Don't worry, you can do it. Dreams do come true so don't ever give up, esp now that you've written a complete story. You owe it to your characters and to that story to keep pushing forward!