Re: copper pipe
Three main things when soldering ( sweating ) copper :
1) clean -- clean -- and clean all the fittings. I like emery cloth to clean the heck out of the exterior of the pipe so it's nice and shinny. For inside fittings there are fine wire brushes used for cleaning those.
2) flux --- make sure to apply a thin coat to the pipe and fittings with flux
3) heat --- have the correct heat applied. If this is your main line there is likely water in the line. You will have a more difficult time to heat the connection enough if you are using a common propane solder flame kit since the water draws the heat away In which case some plumbers will use Mapp gas flame since it burns hotter.
Okay 4 things --- correct solder -- rated for water lines.
"" an ounce of perception -- a pound of obscure "