I have a 50 GB storage plan for a previous phone that I still pay for. How do I make it cover my new phone?
It's assumed you mean an iCloud storage plan and not a storage plan sold/supported by your carrier (i.e. AT&amp;T has Personal Cloud).
iCloud storage is tied to your Apple ID, not to the device itself.  So, all devices that you have signed into with your AppleID, will have access to that storage pool.  Once you sign out, the device will no longer have access to it.  Since you're using a Samsung (Android) device, the only way to access iCloud is via the iCloud Website (https://icloud.com).  If you no longer need this, you simply have to cancel it as there's no Android app for iCloud.
If it is a plan through your carrier, to move your service you would need to contact them for support.