1. Public FolderFirst, you'll need to create a public folder in your Google Drive. That's pretty simple: create a folder, then click on the sharing button, and share it as "public on the web."
2. Folder KeyThe next step might sound confusing, but it's actually not that challenging. You need to get the folder key of the folder you just created. In Google Drive, navigate so that you're looking in the folder you want, then look at the URL in the address bar. The folder key is made up of a bunch of nonsense characters that are the last part of the URL, directly following /#folder/
Copy the folder key by highlighting it and pressing Control-C (or Command-C on a Mac).
Now comes the easy part. Your folder already has a public URL. To find it, type in http://www.googledrive.com/host/ and then paste your folder key directly afterwards. You should end up with something that looks like http://www.googledrive.com/host/0B9exArxMGDDteHNrRHg2bjRHTHc