Your first payment for a consumer proposal depends on a variety of factors.
You may be asked to make your first payment upon filing your insolvency process, to cover the Trustee's costs to register your proposal. These costs include a filing fee with the Superintendent of Bankruptcy, a software license, and sometimes a Court fee, not to mention the hours of work that the Trustee's office spends on the initial consultation, review and preparation of legal documents for your signature.
If you are currently being garnisheed, then typically your first payment will be due within 30 days to give you some time to get funds back in your possession and catch up on rent, utilities, etc.