CCleaner can securely wipe free space: http://filehippo.com/download_ccleaner/
When installed, open it.
Click on "Tools", then click on "Drive Wiper". interface is intuitive & easy to to understand.Oops, just noticed it's an SSD - - not sure you should be wiping it like an HDD so please seek further advice.
I can't help you, I never had an SSD.