Get electronics solder from a supplier like Allied (now RS) https://us.rs-online.com/product/mg-chemicals/4890-18g/70125701/You know what is kind of funny; I am pretty good at welding, but I suck at soldering. Maybe I should find some videos on YouTube and just practice.
Get your iron hot then wipe it clean on a copper scrubby pad then melt a little solder on the tip before you try to heat your connection. You can also wipe it on a damp sponge, but dry copper is better for the tip.
You need a really clean tip and a lot of people have grungy looking stuff that just gets everything hot, but doesn't melt the solder. You might even need a new tip if its all corroded and misshapen.
Wipe tip and apply solder just about every time you are going to do a new connection.
Use the solder "puddle" on the tip to heat the wires, then apply the solder and watch it wick into the joint.
















