Q: Self-Assigned IP Problem for my lan connection on my mac pc (tower)

i have a slight problem here i just bought an apple mac off of my friend and i have tryed to connect an ethernet cable too the tower and it comes up saying self-asigned ip address internet is connected but may not work ? any ideas i have done a few things like:

unclick my proxy ,
manually tryed too put in an ip address ,

restarted my modem ,

and a few other things but still i have had no luck ... help me please ..

i see this is quite a common problem with macs or apples so there must be someone that has abit of knowledge about this .