Play and research with the keyword or phrases
How will people search for any page? Remember that, hardly anyone types and searches only for one word in the search engine. Usually people types 3 or more words in the search engine. When the links introducing your page has the relevant keyword text in them, your page will be ranked higher in search engine results. You have to imagine what are the words people usually type in the search engine. And these words are your target keywords.
Example: you have a page devoted to gardening. When someone types gardening, you want your page to be in the top ten results. This can be your target keyword for that page.
Each page in your web site will have different target keywords that reflect the pages content. Example: you have another page about the position of a garden. Then garden positioning should be the key phrase for the page.