Here is what NOT to do:

Don't just delete your files. Even if you then empty the recycle bin, simple freely available tools can probably recover them.

Don't think that reformatting your disk will wipe it. That only resets the indexes. Most of your data will remain.

Likewise, don't imagine that reinstalling your operating system will do any better. That will only overwrite the start of your disk, leaving most of your data untouched.