my aim is to load with pd sound samples,looping them and play(maybe a far far day) live with my laptop.
now,i've learned to use -loop - object but i have no idea to what could i do now
my question is:
how do you use pd for modifying sounds?(it seems quite a wide question,but i
really dont know where to put my hands now!)

look in the help patches, i think it's the 3rd folder that has a few pitch shift / flanger / filter type things////

i've never used the loop object, so i can't say what it's gonna be like....but if you use something like tabread4~ , and learn how it works by taking a line input. then you can modify that line input to make the sound slow down and speed up and reverse and stuff in realtime.

also, if you can get your head round that, and you've got a fast enough processor...there's heaps of awesome stuff you can do by modifying the help patches for the fft~ stuff.