How do I backup my Galaxy S3 via wifi for free?

[AC Question]

Would like to do this because there's a lot of "junk" thats being stored on the internal hard drive... so I can delete it and free up that space.

 

[hallux]

To clarify for the OP, a "backup" is a second copy of data, stored in a different location from the original, in case the original is lost in some way. Backing up data and then deleting the original is just making a copy elsewhere and having no backup of the file.

Now, Google makes space available in a service called Google Drive. You start with a specified amount of space for free and you can pay for more. If you're just backing up photos, you can back up reduced-resolution versions for free with no hit to your storage limit (but you'll still want the full resolution version someplace). As for backing up app data, that's handled on an app-by-app basis as to how or if they can be backed up.