I'm new to this too and I also have the Nuvi 205. While it's not the best for geocaching it still works decently. Just enter the coordinates (or download the plugin from the Garmin website [free] and upload the cache directly to the unit with a USB cable). BTW, when you use geocaching.com and you search for caches by map, click one of the caches and in the popup you'll see a link that says "Send to GPS". Thats how I upload them.
When you get to the location and the unit says 'Arriving at destination', at the top left it'll show the approximate feet to the location. When you're pretty close, go to the main menu and click Tools -> Where Am I?... This'll show you your exact coordinates, which you can use to reference against the target one.
