i lost my phone and i would like to back up my data

Use a Mac and sign in with Apple ID and password of iPhone at www.icloud.com see the article Archive or make copies of the information you store in iCloud - Apple Support

See the article Use Lost Mode in Find My iPhone on iCloud.com - Apple Support Use Lost Mode in Find My iPhone on iCloud.com - Apple Support

Lost Mode lets you lock your iPhone, iPad, iPod touch, Mac, or Apple Watch so that others can’t access your personal information. You can turn on Lost Mode in Find My iPhone on iCloud.com.

Lost Mode requires a device that meets the minimum system requirements listed in the Apple Support article System requirements for iCloud.

To sign in to Find My iPhone, go to icloud.com/find.

It's too late to do that. You need your device to back it up.