How could I learn how to know where are piano chords like Cm7, D#m6, Amaj9 etc... you know,,those, which have something in the end of it(C,D,E,F,G,A,B) like example "maj6" or "m7b5" :D

The easiest way for this, might be that I could get those from here at the bottom of the page "Piano Chord Chart", but it's a bit obstructive.

I searched from Google for free online lessons, but I didn't find any.

By the way I have played piano now about 3 months.. but I played a year many many years ago, and I started again, and I want to learn more and more

I reckon the best way to learn them is to work them out yourself, working from the original chord.
For example, Dm7b5 is a D minor 7, D F A C, with a flattened 5th. The 5th note in the D minor scale is the A, and so you flatten the A to Ab, and you get D F Ab C. If you try it out with each chord, you should get the hang of it. Do that for each type of thing at the end of a chord, and you should get the hang of all of them and be able to use them easily : ) hope you understand my ramblings :S

It sounds like you just don't know what it MEANS to say C9#5, Dsus4, Fmaj7add2 etc, right? Check out some music theory lessons.

For example:

http://www.dolmetsch.com/introduction.htm
http://www.teoria.com/tutorials/chords/index.htm

Oh, and I believe what the previous poster was talking about with -5 is what's known as a diminished chord (also called a flattened fifth). It refers to an interval of 6 semitones (steps on the chromatic scale) between notes in a chord.