If you are not familiar with Upwork(oDesk), maybe it would be beneficial for you to read Upwork(oDesk) for beginners first.
What factors are most important for customers when they choose a freelancer for their projects: rate, cover letter and feedback.
Of course, your experience is the crucial factor to complete the project successfully, but not for choosing you among other freelancers. When you apply for a job then it automatically means that you have experience to accomplish it.
Rate - you can regulate it as you wish (see how to set up the rate/cost on odesk), but the feedback it is not the thing which you can easily change.
To get a good feedback you must: 1) get a project; 2) complete it successfully.
The cover letter this is how you can explain a customer that you are the best one for this project. See more than hundred real cover letters examples.
The cover letter should show the following:
Your cover letter should include: (simple rules to write great cover letter)