Connect to your SharePoint Online tenant with the SharePoint Online Management Shell.
Start -> All Programs -> SharePoint Online Management Shell then type:
When the Sharepoint online management shell is open and the module is imported we want to connect to our Tenant.
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:
Once that’s completed the Site Collection will be removed and be put in the Site Collection Recycle Bin for 90 days