Those wet solder sponges cause a thermal shock every time you wipe the tip. I recommend using brass windings instead. They also won't completely wipe the solder off the tip either.

You always want a small layer of solder on your tip at all times to prevent it from oxidising. Oxidation means bad heat transfer causing soldering to be much more difficult. Also store it with a small blob of solder on it.

You should avoid scraping or sanding the oxidation of the tip. There is a small layer of silver that won't oxidise so quickly but below that is normal iron. Not sure if that applied to cheaper Chinese irons but it's better not to learn bad habits. Once you remove the silver layer you'll get oxidation much more quickly and you'll have to keep scraping until the tip is gone.

You can remove oxidation with the brass windings much more easily than with a sponge. When you apply solder with flux the rest of the oxidation should come off fairly quickly.

Buying a cheap model to practice is a great idea, when you are used to that you can look into a better iron. I've been very happy with the TS100 but the Pinecil is the newer model apparently.