To solder copper to copper, you need to clean the spots where you want to do the soldering, usually with a fine abrasive. Then you need some solder rosin, preferrably the non-acid type. Rub a small amount on the spot that you cleaned, heat it up with the iron or torch until you can melt the solder a bit on it. Then you can join the 2 pieces with just a bit more solder.
I hope that helps.