First and foremost, you DO NOT send an unsolicited manuscript to anyone. All they’ll do is just throw it in the trash; no reputable company accepts unsolicited material.
Second, you DO NOT contact publishing companies. No reputable publishing firm accepts non-vetted material.
You have two choices: you either indie publish (which won’t cost you a dime if you do all the work yourself – sweat equity – and put the work up yourself on the Holy Trinity of Indie Publishing: Kindle Direct Publishing, Smashwords Direct, and Createspace).; or you find a literary agent.
Finding an agent isn’t easy; you send query letters and hope for the best. Respond (promptly) to the agents who write back, and send them whatever they ask for. Remember, though, that YOU are hiring THEM. You’re the employer, so find the BEST agent to represent you. They’re the ones who will get your work (eventually) in front of a publisher. You’re looking at seeing your work publishing in four years or so if you go this – the traditional – route to getting published.
If, on the other hand, you’d like to be published sooner, have more control over your work, and have it available to the reading publish forever, then indie publish.
As far as getting your work “stolen” … you’re not that good; no reputable firm or agency is going to steal your work. They’re in the business of making money, which they can’t do if they’re thieves; they’d be out of business. If you want to protect your work, then register it through the U.S. Copyright Office. It won’t prevent your work from being stolen, it’ll just give you remedy through the legal system if it does. It’s cheap protection.
To correct misconceptions that will (inevitably) be posted to this question:
- DO NOT send your work to yourself via the mail – certified or registered or whatever – because that is not any type of protection. It’s called the Poor Man’s Copyright, and it’s worthless in court. You can read more about it here: Myth of the Poor Man’s Copyright – Shawn E. Bell.
- DO NOT listen to those who say your work is “copyright as soon as you write it.” Yeah, it is. But without any kind of paper trail and no registration, there’s nothing you can really do to enforce this type of copyright. The people who say this are idiots, and deserve to get their work stolen for their stupidity. Proper registration through the Copyright office gives you additional protections and allows your case to be tried in Federal Court. In other words: you’ll get more money in damages than if you try anything else. Good luck getting a reputable attorney to represent you if your work doesn’t have a proper copyright.
- DO NOT listen to those who say “just put ‘Copyright (c)’ on your work.” That is as bad as – if not worse than – doing both of the above mistakes. Register your work if you’re paranoid about it, but know this: every story has been written. Yours isn’t special to anyone except you.
Finally, you can’t copyright an idea, you can only copyright something that is fixed in tangible form – and ideas aren’t. So don’t give them away.