what type of solder should I use for soldering copper pipes?
You must use lead-free solder meant for water pipes.
Honey gives A-1 advice. I'd only add that after you 'polish' the joint, do NOT touch it to feel if it's smooth. That re-contaminates the joint with finger oils: polish flux ASAP, then proceed.
If the pipes will carry drinking water, use ordinary non-lead wire solder. The keys to soldering copper pipes are: Clean all parts to be soldered with wet/dry sandpaper or a wire brush made especially for cleaning the size of pipe you're dealing with. The pipe needs to be shiny. Smear soldering paste onto both surfaces. Heat with a MATT gas torch until solder melts instantly when touched to the pipe. Do not heat the solder with the torch. Apply solder at one side of the joint. If you have applied enough paste, it will immediately carry the solder all around the joint. Avoid stressing the joint before it is cooled. On any project including multiple joints you should put together the entire assembly complete with cleaned pipe and paste, then proceed to solder the joints from one side of the project to the other.
