Is there a way to connect my iPod touch to the internet without wireless connection (say, by usb)?
Not that I know of however you could try turning internet sharing on on your computer. This will create a wireless network that you can use on your iPod Touch. 

This is easy to do if you are using a Mac. Assuming that internet comes in over Ethernet, go to the Sharing tab in the System Preferences application. Click the internet sharing tick box and configure it to share over Airport. (this is from memory as I don't have access to a Mac right at the moment but you can probably work it out.

Some further details are available here.