There is a lot to know about SEO and many, many companies will scam you. They will claim to get you on first page of google etc.... They will charge month after month till you realize its a scam.
The only way to find a good company is with trustworthy references. (No, I dont mean the ones on the testimonial page that says that sales grew 100000% in a week).
It is almost impossible to "just" hire a SEO company and sit back. You need to do some work yourself or hire someone to do it.
This a popular blog that people discuss SEO on.
http://moz.com/blogGood luck!