Visit your site, log in and then edit your URL to look like this:

https://yourdomain.sharepoint.com/sites/yoursite/yoursubsite/_api/web

It the above URL, the following needs to be replaced:
– replace yourdomain with your domain.
– replace yoursite with your sitename.
– replace yoursite with your subsitename.

For example, if your domain is orecons and your site name is test and subsite is test_subsite then URL will look like:

https://orecons.sharepoint.com/sites/test/test_subsite/_api/web

Or if you don’t have a subsite, use this URL:

https://orecons.sharepoint.com/sites/test/_api/web

Scroll down to the content section and look for the d:Id m:type=”Edm.Guid” element.

Visit your site, log in and then edit your URL to look like this:

The ID of the site should be under d:Id m:type=”Edm.Guid” XML element.

If you have admin rights, you can find the Site ID in the SharePoint admin console. Go to the list of all active sites and click on the site. The URL should change and display the Site ID.