it's very simple actually.
just buy a switch ( or a wifi router in case you want wireless internet on your laptop if it supports wifi )
just connect all three( your modem, pc and laptop ) to the switch using the type of ethernet cables you got with the modem. Now, assign ip addresses to both the laptop and the desktop from the same range as your modem( if your modem ip is a.b.c.d you can assign addresses of type a.b.c.x )
in the gateway ip address type the ip address of the modem ( a.b.c.d ) in the dns type the dns server of your isp ( or use opendns, or your modem's ip if it has a dns server )
subnet mask is : 255.255.255.0
that's it, you are all set