Have a leaking copper pipe at the main.tried to sauder a coupling on the pipeut sauder wouldn't stick.any tios?

First off the pipes must be absolutely clear of all water and moisture. You can not solder with moisture in the pipes.
The secret of soldering is cleaning, fluxing and heat.

Take plumbers sand cloth and shine the pipe back a few inches and with a cleaning brush shine the inside of the fitting.
Now paint both pipe and fitting with NoCorrode Solder paste and join the pipes.
Fire up the torch using a medium tip and apply heat to the BOTTOM of the largest fitting while holding the tip of your solder on TOP.
When the solder begins to melt on top run the tip of the solder around the joint. Capillary action will suck the solder into the joint as you run the solder around the joint. Run the solder around several times and let it set it. You have just completed your solder job. Hope this helps and thank you for rating my reply. Tom

As a compliment to Tom's answer, here's a quick video on the subject:

j1HwfQzxJws

Good luck...

Mark