I've acquired a modest second hand desktop for my son. There's an ethernet socket but it seems no ethernet card so unable to access the internet with it. Is there such a thing as an external ethernet modem/box type thing that might plug into a USB port or something? Or is the only way forward to get someone to put a card in? I'm ok with dvd drives and stuff but would be fearful of trying an ethernet card! Is there a simple gadget I can buy?

If there is an Ethernet socket then there must be a card or more likely it is on the motherboard.

If the socket is there you probably need the drivers. What is the make and model of the computer?

Also just by the way inserting a PCI card is as easy as plugging in anything but again if the socket is there this should be a moot point.

What is the make and model of the computer?

Can you post a picture of the back of the computer so we can look at it?

As Jephree said, you can add a PCI Ethernet card for as little as $5 and there are USB network adapters as well. TigerDirect has one for $19.95 here.