My mac wont connect to the internet without the cable, has happened before and had to have new hardrive and new software. It has happened again and i have tried what people have suggested on other forums but nothing is working.

i have tried everything to get internet to connect but all it comes up with is there is no ip address and wont connect. My iphone connects to the wifi fine, and can use the internet via the cable..

I have been to apple twice now, the last time i had a new harddrive put in and new software installed, the issue is now starting again and its such a hassle..
Does anyone have any quick fix solutions that i can give a go ?
thanks