A few commands here:
1. Check current programme process
$ ps
or with switchers
$ ps -ax

You will get something similar to this:

PID   TTY         TIME      CMD
3511 pts/1        00:00:00  bash
3514 pts/1        00:00:00  ps 

PID is the Process Identification Number. TTY shows which terminal. CMD stands for command.

Linux is real multi-user system, which has got 6 virtual terminals so that 6 user (actually is 7) can login in at the same time. By default, you a login in as TTY0. Press keyboard Ctrl + Alt + F1 (or F1 - F6), monitor shows a black terminal (this is TT1, if you press Ctrl + Alt + F1) asking you type username and password. Switch back to TT0, press Ctrl + Alt + F7.

2. Stop a Process
$ kill -9 3511
(3511 is the PID number)

3. Check Disk Usage
$ df

Filesystem    512-blocks      Free %Used    Iused %Iused Mounted on
/dev/hd1           16384     15368    7%      136     7% /home

or use du command to find out a folder size:
$ du /home (this will list all the size of files in he folder)
$ du -s /home (this will only state folder size)

if you are in the /home directory for example, the directory can be ignored:
$ du -s (same as above)

4. Check memory Usage
$ free
This shows a static memory usage

$ top
This shows a dynamic memory and CPU usage, including all the programs the system is using.


HELP FULL COMMANDs:
if you want to know about meanings of those "-ax" like swichers, type:
$ [the command] --help (very breif manul)

$ man [the command] (very detailed manul, usually has 9 sections)
$ man [section number] [the command]

eg. $ man 5 ps (ps manual section 5)