Hi @wajahat and welcome to the Crossref Community Forum.
In order to register DOIs and deposit metadata for your publications, your organization must be a member of Crossref. You can learn more about applying for membership here.
There are two types of membership: independent membership and sponsored membership. Independent members interact with Crossref directly, depositing their own metadata with us and managing their own quarterly content registration fee and annual membership fee payments. Sponsored members work with an intermediary third-party sponsoring organization who acts as a go-between Crossref and your organization. The sponsor may provide local language support and technical support, and pays Crossref directly; their sponsored members in turn pay the sponsor. You can browse a list of sponsors working in your area here.
I want to emphasize that DOIs are a long-term commitment; while you will pay an initial fee for registering them, you aren’t “buying” them, and you will have an obligation as a Crossref member (either independent or sponsored) to maintain the DOI and the metadata in the long term.
If you have any questions please don’t hesitate to let me know.