what do I do apart from searching on the internet?

On a real UNIX like HP-UX, IRIX, Solaris, FreeBSD or illumos (and any of its distributions), you'd run catman -w as root only once for the lifetime of the system, then run:

  man -k "disk space"

and then you'd search for SEE ALSO with "/", eventually you'd run into df(1), under the very bad presumption that it isn't the first hit.

  EXAMPLE (Solaris 10, identical for illumos / SmartOS)
  > man -k "disk space"
  cfsadmin        cfsadmin (1m)   - administer disk space used for caching file systems with the Cache File-System (CacheFS)
  df              df (1b)         - display status of disk space on file systems
  df_ufs          df_ufs (1m)     - report free disk space on ufs file systems
  snmpdf          snmpdf (1m)     - get a listing of disk space usage on a remote machine by means of SNMP
  space           space (4)       - disk space requirement file
  df              df (1)          - report file system disk space usage
  df              df (1gnu)       - report file system disk space usage

...on a real UNIX, manual pages are THE resource, they are very comprehensive because they were written by professional documentation departments, respectively professional technical writers in those departments working with the actual engineers who developed the software.

The manual page system on a real UNIX is designed to locate exactly the type of information you would be looking for quickly, efficiently and most importantly, consistently.

It's GNU/Linux manual pages that are utter garbage, ergo switch to a system like SmartOS and never look back.