geocaches download

Do I need a premium account to download geocashes. Os is there a website where I can download them.

You can get all geocaches at [Please Register or Login to download file] . There you can load caches onto your device (Garmin Communicator PlugIn needed), without premium account.
If you want to load "pocket queries" (many caches from one region), you need premium acc.

I did download some caches from geocaching.com but when I go into them the device tels me to go premium to get more information.

Don't you get the cache description?

Premium user get the last 5 or 10 user logs, if they want, but AFAIK all other informations get everybody.

I see the description but how can I get it on my device?

If you have a compatible device (e.g. Montana, Oregon, Dakota...), you can download the Communikator plugin: [Please Register or Login to download file] - select Garmin and write it on the connected device.

How can I add dat to a geocache downloaded from geocaching.com. Only premium members can download the gpx file but I want to add the data myself. Is this possible?

When you transfer the cache infos via Communicator plugin on a Garmin device, you have a gpx file in GPX folder. If you are able to edit xml files (possible with every ASCII editor or better with XML editor), you can add every data you want to this file.