Look up the coordinates for the location you want to visit. We select and post caches thru http://www.geocaching.com/.
At http://www.geocaching.com/ you can sign up for a free account that will let you track your visits, log your finds and share your experiences with other geocachers. You'll also be able to search for geocaches worldwide by ZIP code, city, state or keyword.
Once you have the coordinates to your site, plug the information into your GPS unit and off you go on a new adventure.
When you find your cache, follow these simple rules:
i. take something from the cache, leave something in the cache
ii. write about your visit in the logbook -
iii. you can leave an inexpensive trinket such a small toy, fishing lure, souvenir
iv. carefully re-hide the cache for the next person to find
v. take it with you - leave the area in better shape than you found it