We hear from a lot of our clients that they've been contacted by less than reputable SEO companies who can guarantee first page or number 1 spot for them using "link buying" schemes. Our advice is always the same, if its sounds too good to be true, it most likely is.

Part of what gains your site ranking in Google is an analysis by Google of what sites are linking to yours.
The quantity, quality, and relevance of links count towards your rating. There are some SEO companies out there who engage in link exchange schemes which are based on creating a web of links solely for the purpose of manipulating page rank for websites. In the long-term, these schemes will only have a negative impact on the overall ranking of your site.

Google webmaster give advice on the best way to build your website relevance and therefore increase the overall page ranking.

“The best way to get other sites to create relevant links to yours is to create unique, relevant content that can quickly gain popularity in the Internet community. The more useful content you have, the greater the chances someone else will find that content valuable to their readers and link to it.”

Not sure who is linking to your site, use a free tool such as SEO SpyGlass to know just who is linking to your website.