Head over to http://www.stonetrip.com and get a copy of ShiVa3D Web Edition. Currently, there is a download link on the home page. Once you get to the Download page, enter your email address and click on the Download button. If everything goes right, you will be prompted for a save location—save it in a place that will be easy to find later.
That's it for the download, but you may want to take a second to look around Stonetrip's website. There are links to the documentation, forum, wiki, and news updates. It will be well worth your time to become familiar with the site now since you will be using it frequently.