I became liable to pay tax on 1st April, 2017. I have applied for registration on 15th April, 2017 which is within the 30 days’ window given to me. My registration is granted on 29th April, 2017. What document can I issue to collect tax from 1st April to 28th April, 2017?

Till the grant of registration on i.e., 29th April, 2017 tax cannot be collected on the supplies made.
However, even though the registration is granted on 29th April, 2017 the effective date of will be 1st April, as registration is applied for within the permissible period.
Section 31(3)(a) provides for issue of ‘revised invoices’ against the bills raised on a regular basis (without collection of tax) from 1st April to 28th April, within a period of 1 month from the date of grant of registration certificate, i.e., within 29th May, 2017.
Applicable taxes can be collected in the revised invoices issued.

(ICAI FAQ PUBLICATION 06-09-2017 -: Tax Invoice, Credit and Debit Notes : FAQ NO. 16)