Go to findagrave.com and click on [Search 92 million grave records]. (It was only 20 million back in 2008 when I first wrote this! The site is growing fast! If you don't find anything helpful today, check back in a week or two!)
Enter basic information about your ancestor (start with Last Name, year they died, and state, if you know those details) and see what you find! The more unique the name, the less information you should provide in the search screen.
Find A Grave is a volunteer-contributor site. Not every grave in every state is recorded. You may be fortunate and find that your ancestor’s grave has been documented by a kind volunteer. Another way you can be involved in Family History is to BE that kind volunteer! Documenting cemeteries and posting names and headstone photos on Find A Grave would be a great family or Scout service project, or Young Woman’s Value Project.