How can I make sure that my Freelancer gets an initial payment and I pay the rest when the project is completed?

Have you set up a Safepay request for the project? You can negotiate milestones with your freelancer and pay half when part of the work is delivered, and the rest on completion and approval of the full job.
For your protection, I wouldn't recommend that you pay your freelancer funds in advance of any work being provided. As a freelancer, it's great when someone trusts you to that degree, but from a business perspective, it's not a sound practice.

HTH

:)

Use the Safepay service. Once you load it up you can release the payments however you see fit.

The freelancer will have to submit a request for release which you approve.

I would even suggest breaking up the payment into additional releases if this is your first time working with the person. I've done projects that had 3 to 4 milestones to ensure you get the project completed.