Many soldering stations come with a sponge which, when wet, is used to wipe the iron's tip clean. A small amount of fresh solder is usually then applied to the clean tip in a process called tinning.
The copper that forms the heat-conducting bulk of the soldering iron's tip will dissolve into molten solder, slowly eroding the tip. As a result of this, most soldering iron tips are plated to resist wearing down under use. To avoid damaging the plating, abrasives such as sand paper or wire brushes should not be used to clean them. Tips without this plating or where the plating has been broken-through may need to be periodically sanded or filed to keep them smooth.
To avoid using abrasives, cleaning with sal ammoniac is recommended. This comes in a block. You rub the soldering iron bit on the surface. As the surface becomes hot, it begins the cleaning process, noted by the smoke rising from the block. When the block under the bit becomes clear, the bit will be clean and can be tinned as above. If this is done at the end of each session of soldering, the bit will last and will be ready for soldering immediately when you next need to use it.