Originally Posted by: PalsbyYep I know :)
My problem is, that I don't know what kind of adapter I need.
You probably have a 1/4 inch jack in your amp and you are using headphones with a 1/8 inch jack. Therefore you need a 1/8 to 1/4 adapter. These are a couple inches long and fit the headphone jack on one end and the other end plugs into the amp.
They look like this http://www.nextag.com/Hosa-Cable-1-8-530978755/prices-html
That said, I am guessing you are using headphones from your Walkman or something. I have a pair that worked great with my Walkman, but sucked for playing my amp thru. Yours may be fine, but mine sounded buzzy and thin, so I went and bought a new pair with the lowest response threshold I could find. It was worth the $50. (Of course the next week they went on sale for $25 =P )
...so ever since then, I always hang on to the buckle.