If you don't know what Field Day is you owe it to yourself to look it up on the ARRL website.  They say it's not a contest but let's face it there is a points system. You get points for making contacts, you get more points for using CW or other digital modes vs SSB.  If you use an amplifer each contact counts less than if you operate bareboot, or get more points if you go QRP.  The more stations you have the more points you can get and the more visitors and club members can join in the fun.  To find a Field Day site near you just visit the ARRL website and do a search giving your zip code using the Field Day Station Locator.  You will be given a map showing all clubs that have opened their Field Day event to visitors.  Each should have a link to that club's spokesperson.  

If you are new to Amateur Radio or just want to get started look for a site that has a GOTA station listed.  GOTA is a Get On The Air station where they will have a licensed ham to be the control operator and visitors can get on the air even without a license.