Cite facts and figures. Create images of future events which might happen if there is sufficient support. Be very specific. There are a variety of hooks. One is to state that the need is so desperate that your campaign will quite literally grind to a halt if you don't get support.
Desperation works very well, if true.
More positive hooks include the prospect of helping to support something specific. If the reader is still with you by the end of the letter, he knows that you want money. But, he is highly unlikely to respond unless you ask him directly for a specific amount in your concluding sentences. Many first-time fundraisers ask for insignificant amounts because they are afraid of offending someone. This is absolutely wrong.