ż ź dź dż sz cz ś ć - which give most problems to foreigners?

Funny :)

well i can't understand the difference between ź, ż and rz. also the difference between ś and sz, ć and cz, dź and dż. one time someone tried explain me but when she was telling the difference between the letters it was coming me the same anyway :P so i think it needs to live in poland for sometime to understand this.

There are 2 different oral positions (for want of a better term):

Ż and RZ are the same - you know roughly what the sound should be. Put your tongue in about the position as for the English R sound - curled back and pointing at the roof of the mouth.

SZ is the unvoiced version of the same thing, and CZ has the same position.

- and ZI (followed by a vowel) - similar sound, but with the tongue sort of level and pointing forwards to just behind and above the front teeth.

Likewise, Ś and SI (with vowel following) is the same but unvoiced. Ć and CI (followed by a vowel) - again, same position.

And no, I find distinguishing the two different positions in speech difficult too.

some more suggestions:

Ż, RZ - simlar to g in mirage or "si" in vision
-, ZI - similar to French "j" but much softer. In some English dialect it is also similar to the following combination: 'cause you...

Ć - simlar to SPanish "ch" but softer, it's a stop
Ś - very soft sheep, similar to soft frontal ch in some German dialects of ich, it's a fricative and slightly lateral (in contrast standard German ich will require the back of the toungue against the soft palate)

D- - a stop as in John
DŻ - a stop, no similar sounds I can think of. Try to voice the Ć sound. Sometimes can be pronounced as two distinct sounds d + Ż

Actually the only sound that gives me troubles is 'yj' hah, it's not the trouble of saying it but it just feels unnatural when I say it...