Internet, screen and file sharing over USB between two Macs
Best solution would be to either upgrade the iMac to 10.8+ and get WPA2 wifi sharing, or just get any cheap router.
You can't use any sort of regular USB cable for a host-to-host setup, solutions do exist but are definitely a lot more expensive, and much more fiddly software-wise, than either a cheap router or simply a thunderbolt to firewire adapter for your Macbook, which might be your best bet since Macs have native support for sharing internet over firewire.