How would you track DB storage use and then extrapolate it?
You just need to start by collecting data.

There are a few different valid ways of judging the size of the database -- number of rows in each table, size of table and index extents, size of data files -- so first you have to define what decisions you intend to make based on the data. Do you want to know when you'll need to add storage? Or when you'll need to extend data files?