The thing you can spend your money on which would have the biggest effect: condoms (and other contraceptives).

Sounds snarky at first, but really, it isn't. That doesn't mean you shouldn't have any children, but maybe one or two are enough? Also, condoms do not only help you (as in generic you, not OP specifically) in your family planning but may help people who are less well off, too, with the side effect of preventing some STD infections.

Corollary: avoid any charities that oppose directly or indirect family planning and/or contraceptives. That includes anything Catholic and a lot of other Christian charities.