Hi,
to format your local disk, use the installation DVD of new windows, or the same. If you will want to install a new windows, the installation guide will offer you to separate disc, or clean up your disk before install a new version of windows. It’s the best way and your data will be deleted permanently.
Ofc, you can delete your datas manually, but it takes a long time, which you can spend by drinking coffee. Also, if you created recovery point in the last (in the beginning of installation Windows7) you can turn your computer back and then you will lose all your datas - but it may not work.
So, insert installation disc of Windows 10, let the installation guide delete all your datas and software from you HDD and then install a brand new version of Windows 10.