There is NO FREE Windows 10 yet. You must have a preinstalled qualifying Windows, upgrade that for FREE.
After that, you can do a clean install. Again that is on the web so this is the quick answer.
-> But why the 128GB SSD? I find most folk whine about having to work hard to keep it tidy and running out of space.
I have a HP Pavilion dv-7 Laptop. It has two hard drive bays. One is empty and the other has a Toshiba 500 GB Hybrid Hard Drive. I want to add 128 GB SSD and use that for my operating system and programs and reformat the Hybrid Hard Drive and use it for all data storage. I am running Windows 7 Home Edition and plan to install the free upgrade to Windows 10. If I install the new SSD will I be able to install Windows 10 on it or should I install Window 7 first and then upgrade to Windows 10? My concern is that Windows 10 will not install on a new drive that has never had a previous version of Windows.