make sure u use electronics solder also.. some people use plumbers pipe solder.. this does not work well with irons.. it is meant for torches... also i would recomend at least a 40 watt iron with a good size flat tip...
before u solder two items together it is very important to tin both pices.. add solder to the wire till u can see it has flowed all the way through... then when u solder it will bond right..
if you are sokldering to a clean flat surface.. ie battery bars to the cells.. i recomend scuffig the surface.. if it is to smooth solder will not stick.. it needs the scuffing to stick to...