I see my iphone on my computer but still don't kow how to start backup

You can back up your iPhone to your Windows PC using iTunes. If you don't have iTunes installed, you can download it from the Microsoft Store.

Open iTunes and find your device, then click Back Up Now.

Back up your iPhone, iPad or iPod touch in iTunes on PC – Apple Support

Using iTunes, click on the iPhone icon just below the menus:

and on the Summary pane you will find options for both automatic and manual backup: