Well, in principle your kernel should already have the correct module. After plugging it in, try:
dmesg | less (and look for messages about the USB device being plugged in)
You can also see if a device entry had been created:
ls -d /sys/class/net/*
You can attempt to load the module yourself:
sudo modprobe -v rt2x00
It is possible that the module fails to load because the firmware is not available - so check the 'dmesg' log carefully and watch the screen when you attempt to manually load the driver. If the firmware is missing, use the package manager to locate it. In the Debian repository the package name is "firmware-ralink".