The selection of keywords that will trigger the appearance of your charity's ad is vital for the success of your Google AdWords campaign. Think about what words you would type into Google if you were researching a particular issue, or looking for a product or service.
General words or phrases have a higher CPC than specific terms, but don't convert to sales as often. For example, a Google ad for the keywords 'donate charity' costs more than an ad for 'donate children charity', but someone clicking through from the specific search is more likely to donate as they were more certain about what they were looking for.
In other words, having a specific ad will mean your ad isn't displayed as often and will get less clicks in total, but each click should cost you less AND produce better results.
As well as thinking of specific keywords, think of synonyms, spelling variations, plural versions and different verb tenses of your keywords. For example, if you provide housing include synonyms such as 'accommodation', and spelling variations of the latter eg. accomodation, acommodation. Here is a list of commonly mispelled words.
If you're running short of keyword ideas, use Google's Keyword Tool. If you type in a keyword eg. 'recycling' and select the 'Keyword Search Volume' option, it will give you a list of related keywords.
Think creatively about potential keywords. Few people search for a charity to donate to, but many search for information on, say, cancer prevention, or tips on going green. If you provide this information on your website - which you should - then consider linking to it with a Google Ad to raise awareness of your charity.