Put on your safety glasses.

Turn on your soldering iron. If your soldering iron has temperature control:

Set it to 300 °C (~570 °F) if you're using leaded solder

Set it to 375 °C (~700 °F) if you're using lead-free solder

Clean the tip of your soldering iron. If you're using a cellulose sponge, wet the sponge with distilled water until damp and quickly wipe the tip across it. If you're using brass wool, stab the tip into the wire a few times.

Melt a small glob of solder onto the tip of the iron. This is called "tinning the tip" and will help with heat transfer.