Connect to your SharePoint Online tenant with the SharePoint Online Management Shell.

Start -> All Programs -> SharePoint Online Management Shell then type:

Import-Module Microsoft.Online.SharePoint.PowerShell

When the Sharepoint online management shell is open and the module is imported we want to connect to our Tenant.

Connect-SPOService
-Url
https://tenant-admin.sharepoint.com
-Credential
username@yourtenant.onmicrosoft.com

When you are connected the first thing to do is run a Get-SPOSite | select URL

This will get u a list of all SharePoint Online Site Collections

Now we have all the site collections choose your site collection, to remove the site collection type:

Remove-SPOSite https://tenant.sharepoint.com/sites/siteyouwanttoremove

Once that’s completed the Site Collection will be removed and be put in the Site Collection Recycle Bin for 90 days