When you don't want to repeat a noun in your sentence and want to replace it you need to use: one (for only one thing) or ones (for more than one thing).
For example: All the shirts are nice, but I prefer the red one.
I found some interesting new books!
Which ones?
In some sentences signal words can help you:
Signal word for one: another, the, a/an
Signal word for ones: some, these/those