Before you go looking for a letterbox you need to either choose a rubberstamp to use or, even better, design and carve your own. They are easy to make from a rubber eraser. All letterboxes have a rubber stamp and a log book inside them. When you find the letterbox you stamp your stamp into their log book and stamp the one inside the letterbox into your log book. ALWAYS put their rubber stamp back into the box. NEVER take it with you. Then rehide the letterbox exactly where you found it.
Geocaches are different. The absolutely best way to find out about geocaching is to go the site, www.geocaching.com and watch the introduction video.