You'll need:
A set of chalkboard labels - I got mine for around £3 on Ebay:
A chalkboard pen - this cost about £4 from a stationers, but normal chalk would be fine too.
Write out what you want the label to say - you can wipe it off with water and a cloth if you make a mistake:
Then attach your labels to wherever you want to stick them! I chose them to identify these boxes at the top of my wardrobe: