Leaving Germany. How to terminate it??

Hi!

I'm trying to cancel my DSL contract, and I understand that I need some kind of letter of agreement that the company is allowed to charge three monthly fees as a replacement. Where can I find this form?

There's a template?

What does it mean? That after cancellation i pay 3 months?
Or I still get internet and the contract will be terminated in three months?

I don't have the deregistration from Germany. I do have the registration from my new city of residence.  Is that enough? If not, what should I do?

I'm very lost, please help

Best regards

Gali