hi... im a Slackware 9.1 user and i use it with no problems and i got use to it; if i have any problem with something i solve it by googling or asking here...
but, the problem is that i knowspecific things i need to know, for example how to install the program X, or how to use the program Y, but i dont know basic things and command about the system, only what i need at the moment.
for example, today a friend of mine showed me something, and he used lots of commands which i don't know...
i can't specifically tell about the problem here, but the thing i wanna tell here is that i use linux for the specific things i need, and i don't know the system well and it's base commands so that i could handle the system as every good linux user should, and i need people to tell me how to do X or Y, and can't do it myself with my knowledge...
well i hope you understood me, if so, please suggest me waht to do.
thanks!!!!!

Basically, the more you use your system the more familiar everything will become. If you every see a command your not sure about, check its man pages if it has any, or its info pages if it uses that. There are a lot of commands and learning all of them is overkill for most people, but eventually you will pick up everything you need to know. And if you have a problem you don't know how to solve checking here first is a good thing to do. Just remember what was suggested and try to understand what is being suggested, again checking the man pages is great for that.

umm, but there are things i don't know i need, but they are very useful !!!
for example, i didn't know that the command "who", which shows who is logged on the system exists, but it's useful for sure and if my friend wouldn't show it to me i guess i wouldn't be able to know it's even exists !
you know what i mean?
i hope you can help me...

Well, one thing you could do is when you're trying to do something, write a bash script for example, and you need to do a certain task but you're not sure how to then Google for it. Or just ask here for help or an example.
Some of the more useful commands are ps (list running processes), grep (search files for something), tail (very last few lines of a file, namely logs), and the | (pipe). | is very useful because it allows you to chain commands together. As an example, let's say you want to list everything in a directory but it scrolls by too fast for you to see it. You can pipe 'ls -l' to 'more' to get a list of everything in the directory that pauses. Try out this:
Cool, huh?

ohhh i got it! thanks
i will learn bash scripting from tutorials on the net.
maybe you can refer me to a tutorial \ any documentation with the basic and most important unix \ linux commands and other important things i should know?
anyway, i really got the things here and i thank you.

Vlad_m gave you the link for the Linux Documentation project - all the commands will be there. If you did a full install, you will have all the man pages on your system, but you will need to know the command name, and you will need to know this to do scripting. Here is a couple of alphabetical lists that will allow you to become familiar with those commands: http://linux.ctyme.com/man/alpha1.htm

http://www.ss64.com/bash/
I hope this helps...