As long as you approach a reputable company and make sure they have an established track record, they're not going to steal your idea. In addition to being unprofessional, the accusation of theft can destroy a small company like that. You're safe.
And no, you won't be able to get them to sign an NDA. In fact, many companies require
you to sign a form before you submit to them. The forms vary from company to company, and not everyone requires them, but it basically says, "I understand that I'm showing you this of my own free will, you don't owe me anything, and if you happen to be working on a similar project, that's too bad for me."
And BTW, don't pitch an idea. Pitch a
product. Companies don't buy ideas. If you aren't prepared to write the entire project out, don't pitch it. Send them a well written proposal. If you can't write, they don't care how good your idea is.
Follow their guidelines. Most companies put submission guidelines on their web sites. If you don't follow them, you won't be accepted. Ever.
Good luck.
