Pick a trust, a society or a Section 8 company, then register on NGO Darpan and apply in Form 10A for tax registration.

A trust needs at least two trustees, a state-level society seven members, and a Section 8 company two members. Apply in Form 10A at least a month before the tax year you want exemption from; it lasts three years, and 80G approval for donors is a separate application. Foreign donations need FCRA registration.