To fix the WIFI problem, please proceed in these steps until is fixed.
- Check if you have problems with another router. Maybe at the library, at a Starbucks, etc. If you are not having problems, then it is probalby related with your router.
- Reseting the PROM ( http://support.apple.com/kb/ht1379 ) and SMC ( http://support.apple.com/kb/HT3964 ). If does not work try the next one.
- Reset WIFI config: http://osxdaily.com/2012/11/30/resolving-stubborn-wi-fi-connection-problems-in-m ac-os-x/
- The airport card may be not properly connected. See https://www.youtube.com/watch?v=UsSIUs7jzQc
- The airport card may need to be replaced. Check https://www.google.com/search?q=macbook+pro+airport+card to find a replacement. Make sure that it is the model for your laptop. Find here the instructions how to do it: https://www.youtube.com/results?search_query=replace+airport
Note: In steps 3 and 4 you are dealing with hardware problems. An alternative to them is to bring the computer to the Apple Store. These steps can ruin your laptops if you are not good with working with circuits boards. A new airport card is less than the $100 that Apple would ask you but if you break your laptop you have a more than $1,000 loss. Attempt these steps at your own risk.