Problem with IP address

[jelidstone]

When I try to connect to a wireless network. It says there is no IP address? I know nothing about computers. The mac shoppe told me I needed to erase everything and re-install my software. I did this, and it still doesn't work. I'm very frustrated because I lost everything when i did this!
When I connect directly to the modem, it works fine. Please help

 

[TEG]

Check to see that you have your computer setup to use DHCP on your Airport connection. It is also likely that your router is not setup properly. If it is provided by your ISP, contact them, if it is yours, login via an ethernet connection, and change the settings.

TEG