Before thinking charity flying, you have to become a decent pilot with more experience than a PPL license gives you. Think, as a charity flyer you carry responsibility for foreign lifes and you have to become a fairly decent skilled airmen before you do that.
There are organizations around internationally, but typically they want to see at least i.e. >300h PIC, IR and 20h on type last 12 month or at least >200h PIC, IR and 10h on type last 12 month for flying with safety pilot (this example from flyinghope).