Malware

Hello,

I am newish to building wordpress sites, and have just encountered my first malware warning from google on the following site that we build http://www.mmariaswiftinteriors.com/. Webmasters is telling me that this line of code…

<script src="[ link redacted, please don't post that here. ]">

Is located in my root directory and is harmful.

I have two questions…

1. How do I locate and remove this line of code, as I’m not sure where to look…?
2. How can I prevent this from happening in the future?

Any help is greatly appreciated as I don’t know where to go from here.

Many thanks.

Danielle