Most of these sites have a repitation based system, meaning that your chances of being hired are drastically increasing as you complete more jobs.

Generally, from the client's point of view he has two choices:

 1. To hire an **expensive freelancer** with **large reputation**
 2. To hire a **cheap freelancer** with little or **no reputation**

As a new freelancer on the site, you need to try to fall as much as possible in the second target of the client, meaning that offer as low rates as possible. You can gradually increase your rates as you gain reputation and returning clients.