In a terminal window, to see your system resources, use the top command. It shows memory stats, processor stats and running processes.
For disk space use the command df -h. Stands for disk free. The -h shows stats in megabytes or "human readable".
Example screenshot:
Important items are:
Load average, shows the cpu load average for the last 1 min, 5 min, 15 min.
%CPU - not as important as load but worth a look.
%WA - is for WAIT. Meaning how much the cpu is waiting for thing to be written to disk. A busy disk will increase this number.
Kib Mem and Kib Swap. Iinux will use most of memory no matter what, caches as much as possible. Important to watch swap free/used. If a lot of swap is being used, you need more memory.
