How can I purge my Macbook pro in order to improve my storage capacity?

There's only one way: delete user data. Of course if you want to keep the data, you'll need to copy it to an external drive or to a cloud storage system then delete. iCloud is not designed as a backup or storage solution.

Generally the worst "offenders" are pictures, music and other audio-visual files, but masses of user documents ( .doc, .xls and etc) have the same effect.

Apple has guidance on managing space, but if it stays on your drive, it's taking up space.