這句英文怎樣說

Suggest the following formal approach to English speaking foreigners:-

"Good morning (or good afternoon), sir (or miss, madam, etc)

I'm helping this organisation (or say the name of organisation) for their charity fund raising. We would appreciate your donation."

Actually you don't have to say much, because in Chinese, you will say:
請幫我買支旗 , therefore, in English, you can say:
Please donate for charity.

Be precise and to the point, unless they ask you for details.

can you help me to buy a flat？

today is our school (or organization) fund raising. Will u buy a little flag from us? so good fortune may come to you.