Essentially what you're talking about building is a web community of travelers who are willing to share their experiences.
To get advertisers interested in your web site, you'll need to boost your site's membership and traffic. All advertisers want to be on heavily trafficked web sites and sites with lots of members who will see their advertising.
Focus on creating quality content on a regular basis and delivering it through your web site. You might want to pay reviewers a nominal fee at first to get the ball rolling with reviews. Then as you generate interest, people will want to contribute on their own - especially those who have less than pleasant experiences.
This will drive traffic and make people want to participate - which meets your next goal which is to get advertisers.
The other tactic you can employ is studying other sites that have built this kind of community successfully. TripAdvisor.com immediately comes to mind as a company you can study. They don't pay anyone to post reviews but, what started them off on the right foot was giving people access to a wide range of reviews as well as developing their own rating system.
Good luck on your new business.
Lena West is the CEO and Chief Strategist of xynoMedia Technology, a New York-based firm that helps high-growth companies leverage the power of social media, blogs, podcasts and online communities.