The first thing you need is a passport. Fortunately, for travel into Costa Rica, visas aren't necessary. The process of applying for a passport is simple, and almost everything you need is online. Terry did his before traveling to Costa Rica in 2005, and took care of it all at the post office. The photos were at at the higher end of the price range ($8-15), but the quality and convenience were a factor for him. All the information you need is on travel.state.gov.

Once you have everything gathered together, you will need to apply in person at a recognized acceptance location. Again, most post offices can do this, and I found it to be very simple. Other locations can be found on the above pages. You will need your proof of identification (see the guidelines), proof of U. S. citizenship, two 2"x2" photos with a white background (which you can get at Walgreens or wherever - or simply have the post office can do them for you), a completed application, and the fee of $97 total (passport, execution fee, and application fee).

Passports are valid for 10 years, so if you already have one, double-check the expiration date.