I started a subscription with my old apple id and it’s still charging me how can i cancel it ???

You will have to contact Apple and convince them that you really are the owner of that account.

Click here --> https://support.apple.com/choose-country-region Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

Contact Apple for support and service --> "See a list of Apple phone numbers around the world."

Click here --> https://support.apple.com/HT201232

If by "old Apple ID" you mean a different account, sign into that account and cancel it. I you mean you changed Apple IDs on this account, it should appear under your subscriptions and do it there. Either way:

How to cancel a subscription from Apple - Apple Support