Alert: No Internet Connection? MAC

I have a MacBook Pro and Wi-Fi at home but everytime I try to connect it to the Wi-Fi, it doesn't work and an exclaimation mark shows up ontop of my Wi-Fi icon and says "Alert: No Internet Connection". Then I click network preferences and it says "Self assigned ISP" and doesn't let me connect. Sometimes my internet works and sometimes it doesn't. The Wi-Fi works fine on my iphone, ipad, and my sister's PC. Any ideas on how to fix this problem? Please explain to me as simply as you can because I'm no computer expert. Thank you in advanced for helping.

Oh and btw, the network diagnosis doesn't fix the problem.

Does "Sometimes my internet works" mean once a week, once a day or once an hour?
What is the OS version?

"network diagnosis doesn't fix the problem"
Right, not supposed to be a fix, just a diagnosis. The fix will require a change to the router or a change to the OS files… So please tell which OS version… Please. I can hear this: "No way a router can have a problem that is only affecting one computer." Way.