One of the sites like Classmates.com, gradfinder.com, or highschoolalumni.com would be an obvious place to start, but you’d have to have some idea of what school she graduated from, and she’d have to have registered.
Since you said she’s just graduated from high school, she may not have a phone in her own name yet, which would make the phone listings useless.
I’ve had reasonable luck tracking down people I’ve lost touch with using a simple Google.com search. It helps if her name is distinctive, or if you can include some additional keywords you’d associate with her (even if it’s only the state you think she’s in) as additional search terms. If you suspect she’s in Delaware, and might have played volleyball, for example just entering
might turn up a page from her high school web site where she’s mentioned, or a newspaper article, or some other useful information. (Putting the first name and last name together in quotation marks causes the search engine to treat them as a single string, so that only pages with the names together in that order are returned – otherwise, you’ll get pages that simply have the first name and last name anywhere on the page, not necessarily together. One downside to this is if she occasionally uses a middle name or middle initial, and that’s how her name appears on a page, the search will miss that page.)
If that doesn’t turn up anything, try her mother’s name, or her brother’s name.
If something comes up that suggests it might be her, but there’s not enough info to be sure, use keywords associated with the page you find to try the search again.
I’ve been able to find about half the people I’ve looked for this way, but I’ve typically been looking for people who’ve been out of college for fifteen years or so, and who’ve had a lot more time to get their names on lots of web sites.