Q: My computer says I am connected to wifi, but when I go to use the internet, I am not connected. What do I do?

Maybe some help here.

http://support.apple.com/kb/HT1714?viewlocale=en_US

My computer says I am connected to wifi, but when I go to use the Internet, I am not connected.

Usually when this occurs on one Wifi and Internet connection, the wifi router is working, but the router connection to the Internet Service Provider is broken. Resetting the modem or router/modem combination often results in a new connection to the ISP being established (baring any data caps limits, line, modem or ISP troubles)

However your mentioning it's not connecting anywhere your going on wifi or Internet, which could mean your machine has been compromised and possibly blacklisted until it's cleaned up of malware, or you tried to self solve your earlier issues and messed up your Network settings.

I tend to lesser think it's a actual hardware problem, but it might come to that eventually.

You should test the compromised/blacklist theroy by using a Ethernet cable to the modem you know has a good Internet on and powering on/off the modem. (if a router/modem combination, then no need). If you get Internet and not kicked off in a few minutes, then your machine is fine, likely messed up Network settings are too blame.

You can clear everything out by creating a New Location by editing locations and creating a new one, also by going to Airport Advanced > Airport and deleting all the previous connections to start over new. Reboot.

Connect manually to your home network and save in Keychain when it asks.

If this is all too much for you, take it to a local PC/Mac repair shop as they specialize in software/network issues.

From the menu bar, select

 ▹ System Preferences... ▹ Network

Click the Assist me button and select Assistant. Follow the prompts. How far do you get?