When you are ready to solder, plug in your iron and allow the tip to heat up. Periodically take some solder (from your spool) and melt it onto the tip. Leave the solder on the tip until you are ready to solder your connections. When ready, ideally, you should wipe the hot soldering iron tip on a damp sponge to clean it then "tin" the tip by adding a small coating of fresh solder to the tip.
When your soldering iron is sitting idle (hot) you want to add some solder to the tip to always keep it covered. This will preserve the tip and keep it tinned for best heat conductivity when you are soldering. Just make sure to clean the tip of the old solder by wiping on a damp sponge (the tip will be hot and the sponge will hiss, but the old molten solder will come off and collect on the sponge) right before you use the iron to make your solder connections.