You have many choices.
The subject of your question refers to zip files. Do you need to send a compressed (zip) file or are you just interested in encrypting a file to be attached to an email?
If you're concerned about single files, Microsoft Office can password protect Word, Excel, and PowerPoint files: https://support.office.com/en-ie/article/Password-protect-documents-workbooks-and-presentations-ef163677-3195-40ba-885a-d50fa2bb6b68?ui=en-US&rs=en-IE&ad=IE&fromAR=1
If you want to used a compressed file, the version of "zip" included in Windows 7 no longer has the facility to add a password, but 7-zip is an excellent free utility: http://www.7-zip.org/